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This change helps avoid renderer hang crashes on Windows by doing the
potentially costly destruction of a ChunkDemuxer object on a background
task.  The duration of that destruction, not including any initial
scheduling delay, is recorded in a new histogram:
"Media.MSE.DemuxerDestructionTime".

BUG=774268,774288,796704

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I77dcb3ac1f7d5c5777f5302a7fc87199bf693d74
Reviewed-on: https://chromium-review.googlesource.com/838701
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527125}