The MediaSourceState class is currently somewhat misnamed, there is
actually a 1:1 mapping between blink SourceBuffer objects and current
MediaSourceState objects. But a single blink MediaSource can have
multiple SourceBuffers and thus multiple MediaSourceState objects
associated with it. So a better, clearer name for MediaSourceState is
actually SourceBufferState.

Review-Url: https://codereview.chromium.org/2379653004
Cr-Commit-Position: refs/heads/master@{#422234}