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

mkvmuxerutil.cpp

@@ -213,6 +213,8 @@ int32 WriteID(IMkvWriter* writer, uint64 type) {
   if (!writer)
     return -1;
 
+  writer->ElementStartNotify(type, writer->Position());
+
   const int32 size = GetUIntSize(type);
 
   return SerializeInt(writer, type, size);