Add metrics regarding concurrent audible tabs in Chromium.

content/browser/media/audio_stream_monitor.h

Index: content/browser/media/audio_stream_monitor.h
diff --git a/content/browser/media/audio_stream_monitor.h b/content/browser/media/audio_stream_monitor.h
index 0eb51475fbff57067602d16127458f6ab68fe312..12f06055c54970fda0d454eae788c1a6346439aa 100644
--- a/content/browser/media/audio_stream_monitor.h
+++ b/content/browser/media/audio_stream_monitor.h
@@ -52,6 +52,11 @@ class CONTENT_EXPORT AudioStreamMonitor {
   // the killing of tabs making sounds).
   bool WasRecentlyAudible() const;
 
+  // Returns true if the audio is currently audible from the given WebContents.
+  // The difference from WasRecentlyAudible() is that this method will return
+  // false as soon as the WebContents stop producing sound.

[whywhat, 2016/01/15 19:01:20]

hm, won't this create some noise in the metrics? should we count music playlist
as one long audible tab or each track as a separate event?

[mlamouri (slow - plz ping), 2016/01/19 12:18:43]

Sure, this is going to have an effect on playlist. Though, it's better to use
`IsCurrentlyAudible` instead of `WasRecentlyAudible` because otherwise, we will
end up with false negative of concurrent audio output. For example, if I go
pause example.com to play example2.com, we would record a concurrent sound which
should really not be recorded as is.

In the case of playlists, I think we will still see a user with a large number
of concurrent sound. It will be chunks of 3-5 minutes instead of 30+ minutes but
I think we should be able to read the data nonetheless.

+  bool IsCurrentlyAudible() const;
+
   // Starts or stops audio level monitoring respectively for the stream owned by
   // the specified renderer.  Safe to call from any thread.
   //
@@ -139,6 +144,9 @@ class CONTENT_EXPORT AudioStreamMonitor {
   // should be turned on.
   bool was_recently_audible_;
 
+  // Whether the WebContents is currently audible.
+  bool is_audible_;
+
   // Calls Poll() at regular intervals while |poll_callbacks_| is non-empty.
   base::RepeatingTimer poll_timer_;