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

media/cast/test/utility/standalone_cast_environment.h

Index: media/cast/test/utility/standalone_cast_environment.h
diff --git a/media/cast/test/utility/standalone_cast_environment.h b/media/cast/test/utility/standalone_cast_environment.h
index 0deab6dd7c596f9804c6096f96c66b44d6f35022..cf2674e61e7387e8696358955b9752878a1c706c 100644
--- a/media/cast/test/utility/standalone_cast_environment.h
+++ b/media/cast/test/utility/standalone_cast_environment.h
@@ -17,7 +17,7 @@ namespace cast {
 class StandaloneCastEnvironment : public CastEnvironment,
                                   public base::ThreadChecker {
  public:
-  explicit StandaloneCastEnvironment();
+  StandaloneCastEnvironment();
 
   // Stops all threads backing the task runners, blocking the caller until
   // complete.