Fix support for booleans in field trial list parser

The FieldTrialList<T> class hold values internally in a std::vector<T>.
One call site assumes a const T& can be retrieved from the vector, but
it turns out that std::vector<bool> has a bit-packed specialization
which means a normal reference can't be generated.
To work around this, change the code to instead use
typename std::vector<T>::const_reference.

Bug: webrtc:13655
Change-Id: I2a515b7064c8760e1a59d1910d59b1bd2c7a6622
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/250543
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35902}

rtc_base/experiments/field_trial_list.h

@@ -58,7 +58,9 @@ class FieldTrialList : public FieldTrialListBase {
 
   std::vector<T> Get() const { return values_; }
   operator std::vector<T>() const { return Get(); }
-  const T& operator[](size_t index) const { return values_[index]; }
+  typename std::vector<T>::const_reference operator[](size_t index) const {
+    return values_[index];
+  }
   const std::vector<T>* operator->() const { return &values_; }
 
  protected:

rtc_base/experiments/field_trial_list_unittest.cc

@@ -21,13 +21,15 @@ namespace webrtc {
 struct Garment {
   int price = 0;
   std::string color = "";
+  bool has_glitter = false;
 
   // Only needed for testing.
   Garment() = default;
-  Garment(int p, std::string c) : price(p), color(c) {}
+  Garment(int p, std::string c, bool g) : price(p), color(c), has_glitter(g) {}
 
   bool operator==(const Garment& other) const {
-    return price == other.price && color == other.color;
+    return price == other.price && color == other.color &&
+           has_glitter == other.has_glitter;
   }
 };
 
@@ -60,17 +62,20 @@ TEST(FieldTrialListTest, ParsesListParameter) {
 TEST(FieldTrialListTest, ParsesStructList) {
   FieldTrialStructList<Garment> my_list(
       {FieldTrialStructMember("color", [](Garment* g) { return &g->color; }),
-       FieldTrialStructMember("price", [](Garment* g) { return &g->price; })},
+       FieldTrialStructMember("price", [](Garment* g) { return &g->price; }),
-      {{1, "blue"}, {2, "red"}});
+       FieldTrialStructMember("has_glitter",
+                              [](Garment* g) { return &g->has_glitter; })},
+      {{1, "blue", false}, {2, "red", true}});
 
   ParseFieldTrial({&my_list},
                   "color:mauve|red|gold,"
                   "price:10|20|30,"
+                  "has_glitter:1|0|1,"
                   "other_param:asdf");
 
   ASSERT_THAT(my_list.Get(),
-              ElementsAre(Garment{10, "mauve"}, Garment{20, "red"},
+              ElementsAre(Garment{10, "mauve", true}, Garment{20, "red", false},
-                          Garment{30, "gold"}));
+                          Garment{30, "gold", true}));
 }
 
 // One FieldTrialList has the wrong length, so we use the user-provided default
@@ -80,7 +85,7 @@ TEST(FieldTrialListTest, StructListKeepsDefaultWithMismatchingLength) {
       {FieldTrialStructMember("wrong_length",
                               [](Garment* g) { return &g->color; }),
        FieldTrialStructMember("price", [](Garment* g) { return &g->price; })},
-      {{1, "blue"}, {2, "red"}});
+      {{1, "blue", true}, {2, "red", false}});
 
   ParseFieldTrial({&my_list},
                   "wrong_length:mauve|magenta|chartreuse|indigo,"
@@ -88,7 +93,7 @@ TEST(FieldTrialListTest, StructListKeepsDefaultWithMismatchingLength) {
                   "price:10|20|30");
 
   ASSERT_THAT(my_list.Get(),
-              ElementsAre(Garment{1, "blue"}, Garment{2, "red"}));
+              ElementsAre(Garment{1, "blue", true}, Garment{2, "red", false}));
 }
 
 // One list is missing. We set the values we're given, and the others remain
@@ -97,12 +102,13 @@ TEST(FieldTrialListTest, StructListUsesDefaultForMissingList) {
   FieldTrialStructList<Garment> my_list(
       {FieldTrialStructMember("color", [](Garment* g) { return &g->color; }),
        FieldTrialStructMember("price", [](Garment* g) { return &g->price; })},
-      {{1, "blue"}, {2, "red"}});
+      {{1, "blue", true}, {2, "red", false}});
 
   ParseFieldTrial({&my_list}, "price:10|20|30");
 
   ASSERT_THAT(my_list.Get(),
-              ElementsAre(Garment{10, ""}, Garment{20, ""}, Garment{30, ""}));
+              ElementsAre(Garment{10, "", false}, Garment{20, "", false},
+                          Garment{30, "", false}));
 }
 
 // The user haven't provided values for any lists, so we use the default list.
@@ -110,12 +116,12 @@ TEST(FieldTrialListTest, StructListUsesDefaultListWithoutValues) {
   FieldTrialStructList<Garment> my_list(
       {FieldTrialStructMember("color", [](Garment* g) { return &g->color; }),
        FieldTrialStructMember("price", [](Garment* g) { return &g->price; })},
-      {{1, "blue"}, {2, "red"}});
+      {{1, "blue", true}, {2, "red", false}});
 
   ParseFieldTrial({&my_list}, "");
 
   ASSERT_THAT(my_list.Get(),
-              ElementsAre(Garment{1, "blue"}, Garment{2, "red"}));
+              ElementsAre(Garment{1, "blue", true}, Garment{2, "red", false}));
 }
 
 // Some lists are provided and all are empty, so we return a empty list.
@@ -123,7 +129,7 @@ TEST(FieldTrialListTest, StructListHandlesEmptyLists) {
   FieldTrialStructList<Garment> my_list(
       {FieldTrialStructMember("color", [](Garment* g) { return &g->color; }),
        FieldTrialStructMember("price", [](Garment* g) { return &g->price; })},
-      {{1, "blue"}, {2, "red"}});
+      {{1, "blue", true}, {2, "red", false}});
 
   ParseFieldTrial({&my_list}, "color,price");