Add [c]begin() and [c]end() member functions to rtc::Buffer

I can't believe it's been this long before I found out we didn't have
them...

BUG=none

Review-Url: https://codereview.webrtc.org/2804673002
Cr-Commit-Position: refs/heads/master@{#17544}

webrtc/base/buffer.h

@@ -179,6 +179,13 @@ class BufferT {
     return data()[index];
   }
 
+  T* begin() { return data(); }
+  T* end() { return data() + size(); }
+  const T* begin() const { return data(); }
+  const T* end() const { return data() + size(); }
+  const T* cbegin() const { return data(); }
+  const T* cend() const { return data() + size(); }
+
   // The SetData functions replace the contents of the buffer. They accept the
   // same input types as the constructors.
   template <typename U,

webrtc/base/buffer_unittest.cc

@@ -356,6 +356,24 @@ TEST(BufferTest, TestBracketWrite) {
   }
 }
 
+TEST(BufferTest, TestBeginEnd) {
+  const Buffer cbuf(kTestData);
+  Buffer buf(kTestData);
+  auto b1 = cbuf.begin();
+  for (auto& x : buf) {
+    EXPECT_EQ(*b1, x);
+    ++b1;
+    ++x;
+  }
+  EXPECT_EQ(cbuf.end(), b1);
+  auto b2 = buf.begin();
+  for (auto& y : cbuf) {
+    EXPECT_EQ(*b2, y + 1);
+    ++b2;
+  }
+  EXPECT_EQ(buf.end(), b2);
+}
+
 TEST(BufferTest, TestInt16) {
   static constexpr int16_t test_data[] = {14, 15, 16, 17, 18};
   BufferT<int16_t> buf(test_data);