Rework test to avoid playback, since the timing of the opacity
update is unpredictable across platforms and may race with the
test's pixel capture.

--------------[ Original Description Below ]--------------

Same as the last reland but waits for a requestAnimationFrame
cycle to ensure the video surface layer has been painted.

--------------[ Original Description Below ]--------------

Simply moves the win7 expectation to win since they both use the
same one. Formats the HTML and adds a title too. Adds an
expectation for Linux which seems to have changed since last
landing...

--------------[ Original Description Below ]--------------

Apparently we don't have any vpx-alpha layout tests, hence macOS has
had broken support here for some time.

BUG=919015
TEST=fails without the macOS alpha fix.
TBR=dcastagna

Change-Id: I74eb6922e95de8171710f421450e9b69b8e26208
Reviewed-on: https://chromium-review.googlesource.com/c/1480709Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634488}