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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.

Change-Id: I1103f3fd747ca62eff938824bdc0e52899495f64
Reviewed-on: https://chromium-review.googlesource.com/c/1468597Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631877}