Since we never, and never will use a default value of zero
for the alpha channel, we can use a value of zero to
indicate no widening. This has the benefit of not needing
a default value where it does not apply.

BUG=angle:550

Change-Id: Ifdfedcb16039aa1d7f8ee644b65c6b4d89f9a384
Reviewed-on: https://chromium-review.googlesource.com/188432Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>