Bug 881512 - Allow a SourceBuffer to own multiple subdecoders.  Switch decoders on abort().
Bug 881512 - Hide MediaSourceResource implementation details from HTMLMediaElement.
Bug 881512 - Add hidden pref to ignore MIME type in AddSourceBuffer and IsTypeSupported.  For experimental/debugging use only.
Add BufferDecoder::EndDecoding to pair with BeginDecoding.
Start removing mParentDecoder from SubBufferDecoder.
wip