Add metrics regarding concurrent audible tabs in Chromium.

content/browser/media/audio_stream_monitor.h

 // Copyright 2014 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 CONTENT_BROWSER_MEDIA_AUDIO_STREAM_MONITOR_H_
 #define CONTENT_BROWSER_MEDIA_AUDIO_STREAM_MONITOR_H_
 
 #include <map>
 #include <utility>
 
@@ skipping 34 matching lines @@
   static bool monitoring_available() {
     return media::AudioOutputController::will_monitor_audio_levels();
   }
 
   // Returns true if audio has recently been audible from the tab.  This is
   // usually called whenever the tab data model is refreshed; but there are
   // other use cases as well (e.g., the OOM killer uses this to de-prioritize
   // 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.
   //
   // The callback returns the current power level (in dBFS units) and the clip
   // status (true if any part of the audio signal has clipped since the last
   // callback run).  |stream_id| must be unique within a |render_process_id|.
   typedef base::Callback<std::pair<float, bool>()> ReadPowerAndClipCallback;
   static void StartMonitoringStream(
       int render_process_id,
       int render_frame_id,
@@ skipping 67 matching lines @@
   typedef std::map<StreamID, ReadPowerAndClipCallback> StreamPollCallbackMap;
   StreamPollCallbackMap poll_callbacks_;
 
   // Records the last time at which sound was audible from any stream.
   base::TimeTicks last_blurt_time_;
 
   // Set to true if the last call to MaybeToggle() determined the indicator
   // 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_;
 
   // Started only when an indicator is toggled on, to turn it off again in the
   // future.
   base::OneShotTimer off_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioStreamMonitor);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_MEDIA_AUDIO_STREAM_MONITOR_H_