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

content/public/browser/media_observer.h

Index: content/public/browser/media_observer.h
diff --git a/content/public/browser/media_observer.h b/content/public/browser/media_observer.h
index 4bf7c008f375f57fb3562414396aeba6bedff558..c1727dfe638ba922abd33b8a16053f2f2a2e9cad 100644
--- a/content/public/browser/media_observer.h
+++ b/content/public/browser/media_observer.h
@@ -41,6 +41,13 @@ class MediaObserver {
       const MediaStreamDevice& device,
       MediaRequestState state) = 0;
 
+  // Called when an audio stream is played or paused.
+  virtual void OnAudioStreamPlayingChanged(
+      int render_process_id,
+      int render_view_id,
+      int stream_id,
+      bool playing) = 0;
+
  protected:
   virtual ~MediaObserver() {}
 };