Bug 944088 - Make MP4Reader::Init() assume mLayersBackendType=LAYERS_NONE if it doesn't have a decoder owner. r=kinetik

This ensures that if the MP4Reader fails to get a decoder owner, we
don't abort the media load, but instead we will assume that we don't
have an advanced layers backend, and so fallback to software decoding on
Windows.
Bug 943721 - Make MP4Reader restrict its codecs output to H.264 and AAC. r=kinetk

We should restrict the output of MP4Reader to H.264 and AAC so that we
don't end up invoking rogue platform decoders (if that's even possible).
This means that we won't play MP3 audio inside MP4, that's deliberate.