Update media::AudioClock API to take time since writing into account.

media/filters/audio_clock.h

Index: media/filters/audio_clock.h
diff --git a/media/filters/audio_clock.h b/media/filters/audio_clock.h
index a0d8212f94869cd8f929d39831b64e86fb1418bc..625da7d183ee0abc032f116e1dc964b3521587e1 100644
--- a/media/filters/audio_clock.h
+++ b/media/filters/audio_clock.h
@@ -35,7 +35,11 @@ class MEDIA_EXPORT AudioClock {
 
   // Calculates the current media timestamp taking silence and changes in
   // playback rate into account.
-  base::TimeDelta CurrentMediaTimestamp() const;
+  //
+  // Clients can provide |time_since_writing| to simulate the passage of time

[DaleCurtis, 2014/07/15 18:13:15]

Hmm, I don't quite understand how you're going to use this.  I understand what
the unittest is trying to do, but I don't follow how callers will know "time
since writing."  Can you elaborate?

+  // since last writing audio to get a more accurate current media timestamp.
+  base::TimeDelta CurrentMediaTimestamp(
+      base::TimeDelta time_since_writing) const;
 
   // Returns the last endpoint timestamp provided to WroteAudio().
   base::TimeDelta last_endpoint_timestamp() const {