muxer: Reject file if TrackType is never specified

According to matroska specification, TrackType is a mandatory element without a
default value [1]. But we have been producing invalid matroska files when
TrackType is never specified because we write 0 by default.

This patch will reject writing a track without specifying the TrackType value.
Specifically, this path will be triggered when Segment::AddTrack() is called
but there is no subsequent call to Track::set_type().

[1] http://www.matroska.org/technical/specs/index.html#TrackType

Fixes issue #769: https://code.google.com/p/webm/issues/detail?id=769

Change-Id: I4d75d169fd96c7e1fad278561f0e7a3e1135989c

mkvmuxer.cpp

@@ -619,6 +619,10 @@ bool Track::Write(IMkvWriter* writer) const {
   if (!WriteEbmlMasterElement(writer, kMkvTrackEntry, payload_size))
     return false;
 
+  // |type_| has to be specified before the Track can be written.
+  if (!type_)
+    return false;
+
   uint64 size = EbmlElementSize(kMkvTrackNumber, number_);
   size += EbmlElementSize(kMkvTrackUID, uid_);
   size += EbmlElementSize(kMkvTrackType, type_);