Apply minor Media Source Extension spec updates.
Bug 1049317 - Simplify SourceBufferResource::ReadFromCache.
Bug 1049323 - Tidy up SourceBufferResource logging.
Bug 1049323 - Tidy up MSE logging.
Bug 1049326 - Minor MSE cleanups.
Bug 1049327 - Assert that public MSE API is only called from the main thread.
Bug 1049326 - Partially implement EndOfStream.
Bug 1049321 - Move SubBufferDecoder to a new file.
Bug 1049318 - Make SourceBuffer eviction less aggressive.
Bug 1049317 - SBR::ReadFromCache forgot to reset offset.
Bug 1046378 - Handle spurious marker callback in OpenSL backend.
Bug 992238 - Disable test_init_destroy_multiple_contexts_and_streams on Windows 7.
Bug 1050083 - Changes to WebM buffered range handling.

Include the frame duration in the range end.  Also handle (ignore) invalid
negative timestamps when parsing.  Include cluster offset in time/data
offset record.  Cluster offset will be used when calculating discard/resync
points in MSE.
Bug 1050099 - Avoid calling deprecated FindStartTime and instead use buffer's decoded ranges in switching logic.
Bug 1050652 - Implement GetEvictionOffset for WebM reader.
WIP: Improve buffered range calculation for MediaSource objects.