Improve comment around strictly increasing wall clock times.

media/base/time_source.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 MEDIA_BASE_TIME_SOURCE_H_
 #define MEDIA_BASE_TIME_SOURCE_H_
 
 #include <vector>
 
 #include "base/callback.h"
@@ skipping 33 matching lines @@
   // Returns the current media timestamp relative to the timestamp set by
   // SetMediaTime().
   //
   // Values returned are intended for informational purposes, such as displaying
   // UI with the current minute and second count. While it is guaranteed values
   // will never go backwards, the frequency at which they update may be low.
   virtual base::TimeDelta CurrentMediaTime() = 0;
 
   // Converts a vector of media timestamps into a vector of wall clock times. If
   // the media time is stopped, returns false and does not modify the output
-  // vector. Returns true and converts all timestamps otherwise. Guarantees that
-  // wall clock time does not go backwards for monotonically increasing media
-  // timestamps.
+  // vector. Returns true and converts all timestamps otherwise.
+  //
+  // Within a single call to GetWallClockTimes() the returned wall clock times
+  // are a strictly increasing function of the given media times. There is no
+  // such guarantee between calls though; e.g., playback rate or audio delay may
+  // change on other threads within the pipeline.
   //
   // Each timestamp converted from |media_timestamps| will be pushed into
   // |wall_clock_times| such that after all timestamps are converted, the two
   // vectors are parallel (media_timestamps[i] -> wall_clock_times[i]).
   //
   // |media_timestamps| values too far ahead of the current media time will
   // be converted to an estimated value; as such, these values may go backwards
   // in time slightly between calls to GetWallClockTimes().
   //
   // |media_timestamps| values behind the current media time may be
   // significantly incorrect if the playback rate has changed recently. The only
   // guarantee is that the returned time will be less than the current wall
   // clock time.
   virtual bool GetWallClockTimes(
       const std::vector<base::TimeDelta>& media_timestamps,
       std::vector<base::TimeTicks>* wall_clock_times) = 0;
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_TIME_SOURCE_H_