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

chrome/browser/media/audio_stream_indicator.h

Index: chrome/browser/media/audio_stream_indicator.h
diff --git a/chrome/browser/media/audio_stream_indicator.h b/chrome/browser/media/audio_stream_indicator.h
new file mode 100644
index 0000000000000000000000000000000000000000..92097493b3fde77f790b49b8d8035ccafcd5d085
--- /dev/null
+++ b/chrome/browser/media/audio_stream_indicator.h
@@ -0,0 +1,40 @@
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_MEDIA_AUDIO_STREAM_INDICATOR_H_
+#define CHROME_BROWSER_MEDIA_AUDIO_STREAM_INDICATOR_H_
+
+#include <set>
+
+#include "base/memory/ref_counted.h"
+
+namespace content {
+class WebContents;
+}
+
+class AudioStreamIndicator
+    : public base::RefCountedThreadSafe<AudioStreamIndicator> {
+ public:
+  AudioStreamIndicator();
+
+  // This method should be called on the IO thread.
+  void UpdateWebContentsStatus(int render_process_id,
+                              int render_view_id,

[miu, 2012/12/19 00:49:08]

Please fix indentation here.

[Bernhard Bauer, 2013/02/05 18:45:18]

Done.

+                              bool playing);
+
+  // This method should be called on the IO thread.
+  bool IsPlayingAudio(content::WebContents* contents);
+
+ private:
+  friend class base::RefCountedThreadSafe<AudioStreamIndicator>;
+  virtual ~AudioStreamIndicator();
+
+  void UpdateWebContentsStatusOnUIThread(int render_process_id,
+                                        int render_view_id,

[miu, 2012/12/19 00:49:08]

Ditto (fix indentation).

[Bernhard Bauer, 2013/02/05 18:45:18]

Done.

+                                        bool playing);
+
+  std::multiset<std::pair<int, int> > audio_streams_;

[miu, 2012/12/19 00:49:08]

Please document this member.  I assume it's the set of playing audio streams?

[miu, 2012/12/19 00:49:08]

Why is this a multiset<>?  Won't a set<> do the job?

[tommi (sloooow) - chröme, 2012/12/19 11:47:29]

Instead of using std::pair, let's just define a proper struct for this with
render_process_id and render_view_id members.  
std::pair just makes the code harder to read if you have to remember what
'first' and 'second' is and it's easy to mix them up.

[Bernhard Bauer, 2013/02/05 18:45:18]

Done.

[Bernhard Bauer, 2013/02/05 18:45:18]

Done.

[Bernhard Bauer, 2013/02/05 18:45:18]

It's a multiset because it has one entry per audio stream in a render view.

+};
+
+#endif  // CHROME_BROWSER_MEDIA_AUDIO_STREAM_INDICATOR_H_