Bug 991448 - Skip Theora decode to next keyframe after seek, so that we don't get visual artifacts after a fastSeek. r=cajbir
Bug 986947 - Make MP3 contained in MP4 playback again on Windows with WMF backend. r=padenot

Report that we can play MP3 inside MP4 on Windows Vista and later in
HTMLMediaElement.canPlayType. Chrome and IE on Windows match this behaviour.

Add a test file with MP3 contained inside MP4. Note the B2G emulator can't play
this file, so I added a codecs parameter to the file's mime type so that
decoder backends have to opt-in to testing with it.

Enable playback of MP3 inside MP4 in WMFReader.

Change from reporting the IMFSourceReader's duration inside WMFReader, to
instead report the IMFSourceReader's duration as the media "end time". This is
needed because the MP3-contained-in-MP4 file's first sample output by the
IMFSourceReader has a non-zero timestamp, and the MediaDecoderStateMachine
assumes that the media samples will be in the range
[$firstSampleStartTime, $firstSampleStartTime+$reportedDuration]. But that's
not the case here, the IMFSourceReader seems to output samples in the range
[0,$reportedDuration]. This assumption mismatch means on the
MP3-contained-in-MP4 file we end up trying to seek after what the
IMFSourceReader assumes is the end of the file, which fails and causes
test failures.