Turn audio ducking on by default on Windows again.

content/renderer/media/media_stream_dispatcher.h

Index: content/renderer/media/media_stream_dispatcher.h
diff --git a/content/renderer/media/media_stream_dispatcher.h b/content/renderer/media/media_stream_dispatcher.h
index b7c666080fa4c47c9ddcc7969b3a46f25473a9ed..59363a3e0b9be401dc4da0c530657947e2513872 100644
--- a/content/renderer/media/media_stream_dispatcher.h
+++ b/content/renderer/media/media_stream_dispatcher.h
@@ -95,6 +95,11 @@ class CONTENT_EXPORT MediaStreamDispatcher
   // Returns an audio session_id given a label and an index.
   virtual int audio_session_id(const std::string& label, int index);
 
+  // Returns true if an audio input stream is currently active that was opened
+  // with audio ducking enabled.  This is information is used when playing out
+  // audio so that rendered audio can be excluded from the ducking operation.
+  bool IsAudioDuckingActive() const;
+
  protected:
   int GetNextIpcIdForTest() { return next_ipc_id_; }
 
@@ -102,6 +107,7 @@ class CONTENT_EXPORT MediaStreamDispatcher
   FRIEND_TEST_ALL_PREFIXES(MediaStreamDispatcherTest, BasicVideoDevice);
   FRIEND_TEST_ALL_PREFIXES(MediaStreamDispatcherTest, TestFailure);
   FRIEND_TEST_ALL_PREFIXES(MediaStreamDispatcherTest, CancelGenerateStream);
+  FRIEND_TEST_ALL_PREFIXES(MediaStreamDispatcherTest, CheckDuckingState);
 
   struct Request;