Fix race on demuxer memory usage. Reuse previous calculation.

media/base/demuxer.h

 // Copyright (c) 2012 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_DEMUXER_H_
 #define MEDIA_BASE_DEMUXER_H_
 
 #include <vector>
 
 #include "base/time/time.h"
@@ skipping 69 matching lines @@
   virtual void Stop() = 0;
 
   // Returns the starting time for the media file; it's always positive.
   virtual base::TimeDelta GetStartTime() const = 0;
 
   // Returns Time represented by presentation timestamp 0.
   // If the timstamps are not associated with a Time, then
   // a null Time is returned.
   virtual base::Time GetTimelineOffset() const = 0;
 
-  // Returns the memory usage in bytes for the demuxer.
+  // Returns the memory usage in bytes for the demuxer. May be called from any
+  // thread.
   virtual int64_t GetMemoryUsage() const = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Demuxer);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_DEMUXER_H_