Bug 739094 - Push an empty AudioData instance to the AudioQueue when first WebM audio packet is empty.  r=cpearce

Without this, the media start time is derived from the first packet that produces audio, but that may have a non-zero start time in badly muxed files.
Bug 1072483 - Fix unit conversion error in WebMReader::GetBuffered.  r=cpearce
Bug 1072483 - Restore cluster sync search to WebM buffered parser (from bug 1044498) and use it when starting a parser from an arbitrary offset.  r=cajbir