Take missed opportunity: memset for A8.

Just recently noticed we weren't doing this.
It seems to be triggering all the time.

Draws the same of course.

Change-Id: Ie3d869a05194a5d52d635643eba1c96c8a21ce67
Reviewed-on: https://skia-review.googlesource.com/8471
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed