Bug 945085 - Improve comments in PlatformDecoderModule.h. r=kinetik

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.
Bug 945947 - Re-enable DirectShow for MP3 playback on Vista and later. r=ajones

We disabled MP3 playback on Windows using DirectShow as the duration was
being incorrectly reported. While DirectShow was disabled, we supported
MP3 playback via Windows Media Foundation, and we still used DirectShow
on WinXP. Now that bug 918135 has landed the duration calculation has
been fixed, so we can re-enable DirectShow for MP3 playback for all
Windows versions. This enables us to avoid suffering from bug 882537 for
MP3 on Windows.