Fix muxer output with no Cues.

- Fixed a bug with the muxer where it would generate a entry in
  the SeekHead for a Cues element with OutputCues set to false.
- Fixed a bug with the muxer where it would generate an empty
  Cues element with OutputCues set to false.
- Issue:http://code.google.com/p/webm/issues/detail?id=383

Change-Id: I99cc404a7e8ad61759dbb57ab769625dd724240c
1 file changed