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.
Bug 881512 - Start processing multiple decoders.
Bug 881512 - Switch video readers on EOF.
Bug 881512 - Crude attempt at segment parser for WebM.
Bug 881512 - Unpref Media Source Extensions (for WebM) in non-release builds.
Temp fix: don't call mParentDecoder functions while SBD runs off decode thread.  Revert when SBD runs on state machine's task queue.
Don't pretend the entire resource is always completely cached.
Use the base video and audio queues.  Currently works by shifting media data
from the sub-reader queues on each decode call.  Future work could allow the
sub-readers to use the base queues directly.