mkvmuxer: Add element start notification.

Just before the first byte of an element identifier is written
the muxer calls IMkvWriter::ElementStartNotify to report the
position of the element in the WebM stream.

Change-Id: Iac40090587bd9496b05f41203aace00902f5606e

mkvwriter.hpp

@@ -26,6 +26,7 @@ class MkvWriter : public IMkvWriter {
   virtual int64 Position() const;
   virtual int32 Position(int64 position);
   virtual bool Seekable() const;
+  virtual void ElementStartNotify(uint64 element_id, int64 position);
   virtual int32 Write(const void* buffer, uint32 length);
 
   // Creates and opens a file for writing. |filename| is the name of the file