Add a method to tab_utils.h to find out whether a tab is playing audio.

content/browser/renderer_host/media/mock_media_observer.h

Index: content/browser/renderer_host/media/mock_media_observer.h
diff --git a/content/browser/renderer_host/media/mock_media_observer.h b/content/browser/renderer_host/media/mock_media_observer.h
index 3c97f0a994f6c525ef9b573a971fcad015959939..3bd5bf6cda280e7ef32fa4636afc7564aa679786 100644
--- a/content/browser/renderer_host/media/mock_media_observer.h
+++ b/content/browser/renderer_host/media/mock_media_observer.h
@@ -34,6 +34,10 @@ class MockMediaObserver : public MediaObserver {
                void(int render_process_id, int render_view_id,
                     const MediaStreamDevice& device,
                     const MediaRequestState state));
+  MOCK_METHOD3(OnAudioStreamPlayingChanged,
+               void(int render_process_id,
+                    int render_view_id,
+                    bool playing));
 };
 
 class MockMediaInternals : public MediaInternals {