Enable round-tripping and updating of SourceBuffer timestamp offset

public/platform/WebSourceBuffer.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 31 matching lines @@
         AppendModeSequence
     };
 
     virtual ~WebSourceBuffer() { }
     virtual bool setMode(AppendMode) = 0;
     virtual WebTimeRanges buffered() = 0;
     virtual void append(const unsigned char* data, unsigned length) = 0;
     virtual void abort() = 0;
     virtual void remove(double start, double end) = 0;
     virtual bool setTimestampOffset(double) = 0;
+    virtual double getTimestampOffset() = 0;
 
     // Set presentation timestamp for the start of append window.
     virtual void setAppendWindowStart(double) = 0;
 
     // Set presentation timestamp for the end of append window.
     virtual void setAppendWindowEnd(double) = 0;
 
     virtual void removedFromMediaSource() = 0;
 };
 
 } // namespace blink
 
 #endif