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This fixes a regression introduced after [1].  The issue is that there
may not always be an alpha visual.

[1] https://chromium.googlesource.com/chromium/src/+/76ddcd29071bd4079c8776e0576268da5493f3ff

BUG=1001026
R=kbr

Change-Id: I040082e9e1921c5a541aecb80279ca7c50275f71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906318
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714054}