[Cast] Remove AudioDecoder's dependency on WebRTC, and refactor/clean-up AudioReceiver.

media/cast/test/utility/audio_utility.cc

Index: media/cast/test/utility/audio_utility.cc
diff --git a/media/cast/test/utility/audio_utility.cc b/media/cast/test/utility/audio_utility.cc
index f8e4957b7c35f585ef6ec4bf6a4b6850d903df93..b1c01909bbc7baa24d77f23da628a4bd5e40cfa0 100644
--- a/media/cast/test/utility/audio_utility.cc
+++ b/media/cast/test/utility/audio_utility.cc
@@ -53,6 +53,26 @@ scoped_ptr<PcmAudioFrame> ToPcmAudioFrame(const AudioBus& audio_bus,
   return audio_frame.Pass();
 }
 
+int CountZeroCrossings(const float* samples, int len) {
+  // The sample values must pass beyond |kAmplitudeThreshold| on the opposite
+  // side of zero before a crossing will be counted.
+  const float kAmplitudeThreshold = 0.03f;  // 3% of max amplitude.
+
+  int count = 0;
+  int i = 0;
+  float last = 0.0f;
+  for (; i < len && fabsf(last) < kAmplitudeThreshold; ++i)
+    last = samples[i];
+  for (; i < len; ++i) {
+    if (fabsf(samples[i]) >= kAmplitudeThreshold &&
+        (last < 0) != (samples[i] < 0)) {
+      ++count;
+      last = samples[i];
+    }
+  }
+  return count;
+}
+
 int CountZeroCrossings(const std::vector<int16>& samples) {
   // The sample values must pass beyond |kAmplitudeThreshold| on the opposite
   // side of zero before a crossing will be counted.