ChunkDemuxerStream CL: https://chromium-review.googlesource.com/c/chromium/src/+/1279242

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a0810e4e9e8ffa5475acd49373367423d64e1740

commit a0810e4e9e8ffa5475acd49373367423d64e1740
Author: Matt Wolenetz <wolenetz@chromium.org>
Date: Sat Oct 13 00:55:37 2018

MSE: Annotate thread-safetiness related to ChunkDemuxerStream::lock_

Adds GUARDED_BY and related annotations to ChunkDemuxerStream.
Adding the annotations makes it clear which fields are protected by
ChunkDemuxerStream::lock_ and enables compile-time verification.

These additions confirmed previous lack of appropriate locking in four
ancillary ChunkDemuxerStream methods; this CL also fixes those cases.

Credit to jrummell@ for identifying the missing locking, and to
chcunningham@ for promoting the new static annotations' usage.

BUG=895017,887645
R=jrummell@chromium.org

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I0c14ac2fedf60df84e616cf3c268b876d8f8c514
Reviewed-on: https://chromium-review.googlesource.com/c/1279242
Reviewed-by: John Rummell <jrummell@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599452}
[modify] https://crrev.com/a0810e4e9e8ffa5475acd49373367423d64e1740/media/filters/chunk_demuxer.cc
[modify] https://crrev.com/a0810e4e9e8ffa5475acd49373367423d64e1740/media/filters/chunk_demuxer.h