Bug 1091008 - Factor out IsDataCachedToEndOfResource() into a higher-level question and answer it sensibly for MSE. v1 r=cpearce
Bug 1091008 - Teach MediaDecoderReader about its start time. v2 r=cpearce

Duplicating state is never great, but this lets the reader make calculations
using this immutable state variable without involving the state machine. We
could alternatively punch a hole from MediaDecoderReader to
MediaDecoderStateMachine and access it there, but that would create tighter
coupling, and weird relationships for MSE.
Bug 1091008 - Remove the aStartTime argument from MediaDecoderReader::GetBuffered. v1 r=cpearce

We now have this stashed on the superclass.
Bug 1091008 - Hoist stream pinning into the MediaDecoderReaders and make MediaDecoderStateMachine::GetBuffered just forward to them. v1 r=cpearce

Whether or not we ping a stream depends a lot on what kind of decoder we're
dealing with. In particular, it doesn't really make sense for MSE.
Bug 1091008 - Implement a sensible GetBuffered override for MediaSourceReader using the existing GetBuffered on MediaSource. v1
Bug 1091008 - Remove special handling for MSE in HTMLMediaElement::Buffered. v1

The same code will now be reached by invoking this method on the decoder.
Bug 1091008 - Reimplement HasLowUndecodedData in terms of GetBuffered. v2

Since GetBuffered now has a sane implementation for MSE, this should
make this function sane for MSE as well.
Bug 1063323 - Tests. v3