Mark all bool conversion operators as explicit

An explicit bool conversion operator will still be used implicitly
when an expression appears in "bool context", e.g., as the condition
in an if statement, or as argument to logical operators. The
`explicit` annotation prevents conversion in other contexts, e.g.,
converting both a and b to bool in an expression like `a == b`.

Bug: None
Change-Id: I79ef35b1ea831e6011ae472900375ae8a3e617ab
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/250664
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35927}

logging/rtc_event_log/events/rtc_event_field_encoding_parser.h

@@ -36,7 +36,9 @@ class RtcEventLogParseStatus {
   }
 
   bool ok() const { return error_.empty(); }
-  ABSL_DEPRECATED("Use ok() instead") operator bool() const { return ok(); }
+  ABSL_DEPRECATED("Use ok() instead") explicit operator bool() const {
+    return ok();
+  }
 
   std::string message() const { return error_; }
 

modules/congestion_controller/goog_cc/probe_controller.cc

@@ -225,7 +225,7 @@ std::vector<ProbeClusterConfig> ProbeController::OnMaxTotalAllocatedBitrate(
         probes.push_back(second_probe_rate.bps());
     }
     return InitiateProbing(at_time_ms, probes,
-                           config_.allocation_allow_further_probing);
+                           config_.allocation_allow_further_probing.Get());
   }
   max_total_allocated_bitrate_ = max_total_allocated_bitrate;
   return std::vector<ProbeClusterConfig>();

modules/desktop_capture/linux/wayland/shared_screencast_stream.cc

@@ -149,7 +149,7 @@ class ScopedBuf {
     }
   }
 
-  operator bool() { return map_ != MAP_FAILED; }
+  explicit operator bool() { return map_ != MAP_FAILED; }
 
   void initialize(uint8_t* map, int map_size, int fd) {
     map_ = map;

rtc_base/experiments/field_trial_parser.h

@@ -220,7 +220,7 @@ class FieldTrialFlag : public FieldTrialParameterInterface {
   explicit FieldTrialFlag(std::string key);
   FieldTrialFlag(std::string key, bool default_value);
   bool Get() const;
-  operator bool() const;
+  explicit operator bool() const;
 
  protected:
   bool Parse(absl::optional<std::string> str_value) override;

sdk/objc/unittests/scoped_cftyperef_tests.mm

@@ -16,7 +16,7 @@ namespace {
 struct TestType {
   TestType() : has_value(true) {}
   TestType(bool b) : has_value(b) {}
-  operator bool() { return has_value; }
+  explicit operator bool() { return has_value; }
   bool has_value;
   int retain_count = 0;
 };