Add audio playback (all platforms) to cast_receiver_app.

media/cast/test/utility/in_process_receiver.h

Index: media/cast/test/utility/in_process_receiver.h
diff --git a/media/cast/test/utility/in_process_receiver.h b/media/cast/test/utility/in_process_receiver.h
index de18845bfbcad04d12713bcc795dd232ad441a77..f5624ced281a932e7efed15b1ced51b5d53e4b37 100644
--- a/media/cast/test/utility/in_process_receiver.h
+++ b/media/cast/test/utility/in_process_receiver.h
@@ -59,7 +59,7 @@ class InProcessReceiver {
 
   // Begin delivering any received audio/video frames to the OnXXXFrame()
   // methods.
-  void Start();
+  virtual void Start();
 
   // Schedules destruction on the cast MAIN thread.  Any external references to
   // the InProcessReceiver instance become invalid.
@@ -69,7 +69,7 @@ class InProcessReceiver {
 
   // Destroy the sub-compontents of this class.
   // After this call, it is safe to destroy this object on any thread.
-  void Stop();
+  virtual void Stop();
 
  protected:
   // To be implemented by subclasses.  These are called on the Cast MAIN thread