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This is part 1/4 CLs to move the <audio>/<video> elements off of
a synchronous API that can lead to renderer hangs and premature
audio renderer errors.

BUG=905506
TEST=updated tests, compiles.

Change-Id: I9b26ed5ed8faea2f691b3a415dbdc79e18f5125a
Reviewed-on: https://chromium-review.googlesource.com/c/1347588
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612330}