GitLab

The crash happens because by the time we call Demuxer::GetStream again
the main thread might have proceeded with destroying SourceBuffer and
thus ChunkDemuxer no longer allows accessing demuxer streams. Luckily
demuxer streams themselves are still alive, just moved to
ChunkDemuxer::removed_streams_ (see ChunkDemuxer::RemoveId). So cached
pointers can still be used safely.

BUG=668604

Review-Url: https://codereview.chromium.org/2558213002
Cr-Commit-Position: refs/heads/master@{#438000}