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From video_decode_accelerator.h, it's not clear if it's valid to
call Reset before NotifyFlushDone. Currently ARC may call Reset
before NotifyFlushDone is back. We need to document this and add
a test for it.

BUG=671944
TEST=None

Review-Url: https://codereview.chromium.org/2556873003
Cr-Commit-Position: refs/heads/master@{#438774}