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

content/browser/renderer_host/media/audio_renderer_host.cc

Index: content/browser/renderer_host/media/audio_renderer_host.cc
diff --git a/content/browser/renderer_host/media/audio_renderer_host.cc b/content/browser/renderer_host/media/audio_renderer_host.cc
index 1901e2fb1ebf9b1cfbfe277d65313dcaff60b85a..b4cd34e324e1d36a7af7e811aef2735159d4006c 100644
--- a/content/browser/renderer_host/media/audio_renderer_host.cc
+++ b/content/browser/renderer_host/media/audio_renderer_host.cc
@@ -13,6 +13,8 @@
 #include "content/browser/renderer_host/media/audio_mirroring_manager.h"
 #include "content/browser/renderer_host/media/audio_sync_reader.h"
 #include "content/common/media/audio_messages.h"
+#include "content/public/browser/content_browser_client.h"
+#include "content/public/browser/media_observer.h"
 #include "media/audio/shared_memory_util.h"
 #include "media/base/audio_bus.h"
 #include "media/base/limits.h"
@@ -331,6 +333,9 @@ void AudioRendererHost::OnAssociateStreamWithProducer(int stream_id,
   if (entry->render_view_id == render_view_id)
     return;
 
+  // TODO(miu): Merge "AssociateWithProducer" message into "CreateStream"
+  // message so AudioRendererHost can assume a simpler "render_view_id is set
+  // once" scheme. http://crbug.com/166779
   if (mirroring_manager_) {
     mirroring_manager_->RemoveDiverter(
         render_process_id_, entry->render_view_id, entry->controller);
@@ -354,6 +359,13 @@ void AudioRendererHost::OnPlayStream(int stream_id) {
   entry->controller->Play();
   if (media_internals_)
     media_internals_->OnSetAudioStreamPlaying(this, stream_id, true);
+
+  MediaObserver* media_observer =
+      GetContentClient()->browser()->GetMediaObserver();
+  if (media_observer) {
+    media_observer->OnAudioStreamPlayingChanged(
+        render_process_id_, entry->render_view_id, true);
+  }
 }
 
 void AudioRendererHost::OnPauseStream(int stream_id) {
@@ -368,6 +380,13 @@ void AudioRendererHost::OnPauseStream(int stream_id) {
   entry->controller->Pause();
   if (media_internals_)
     media_internals_->OnSetAudioStreamPlaying(this, stream_id, false);
+
+  MediaObserver* media_observer =
+      GetContentClient()->browser()->GetMediaObserver();
+  if (media_observer) {
+    media_observer->OnAudioStreamPlayingChanged(
+        render_process_id_, entry->render_view_id, true);

[miu, 2013/02/05 23:18:25]

I think you meant s/true/false/ here.

[miu, 2013/02/05 23:18:25]

Also, note that streams are not necessarily paused before they are closed (or
error out).  You'll somehow need to call
MediaObserver::OnAudioStreamPlayingChanged(..., false) in those cases.  Probably
in AudioRendererHost::CloseAndDeleteStream().

...and the unfortunate news here is: How will you balance the number of
OnAudioStreamPlayingChanged(true) with OnAudioStreamPlayingChanged(false) calls?

[Bernhard Bauer, 2013/02/08 16:44:14]

Urr, yes. Done.

[Bernhard Bauer, 2013/02/08 16:44:14]

Hm. I changed it to a map from RenderView ID to a set of streams playing.

+  }
 }
 
 void AudioRendererHost::OnFlushStream(int stream_id) {