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

mkvmuxer.hpp

@@ -35,6 +35,13 @@ class IMkvWriter {
   // Returns true if the writer is seekable.
   virtual bool Seekable() const = 0;
 
+  // Element start notification. Called whenever an element identifier is about
+  // to be written to the stream. |element_id| is the element identifier, and
+  // |position| is the location in the WebM stream where the first octet of the
+  // element identifier will be written.
+  // Note: the |MkvId| enumeration in webmids.hpp defines element values.
+  virtual void ElementStartNotify(uint64 element_id, int64 position) = 0;
+
  protected:
   IMkvWriter();
   virtual ~IMkvWriter();

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);

mkvwriter.cpp

@@ -88,4 +88,7 @@ bool MkvWriter::Seekable() const {
   return true;
 }
 
+void MkvWriter::ElementStartNotify(uint64, int64) {
+}
+
 }  // namespace mkvmuxer

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