Adds comments to PlatformDecoderModule.h, and describes the call model
better. Renames CreateAudioDecoder and CreateVideoDecoder to
CreateAACDecoder and CreateH264Decoder. We may change them to accept a
codec parameter and have more general names in future when we need to
support more codecs here.
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.
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.
