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VEA flush support isn't known until Initialize() completes, which
happens by way of RequireBitstreamBuffers(). The old code uses the
value of |flush_support_| during InitCompleted() despite it not
being set until after InitCompleted() runs.

The fix is to ensure we set |flush_support_| correctly before we
use it and to switch to a base::Optional for its value to prevent
accidental incorrect usage in the future.

R=sandersd

Fixed: 1163673
Test: New unittest.
Change-Id: I69cec2f954ffc68773984ab59227b2cd10079820
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613670
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840877}