Revert "Take missed opportunity: memset for A8."

Revert "Take missed opportunity: memset for A8."

This reverts commit 9cee33e631679fc8195e7892b5aa5fdffb0b568e.

Reason for revert: Chrome roll?

Original change's description:
> 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>
>

TBR=mtklein@chromium.org,msarett@google.com,reed@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I26a3f2b1a9ae9093180e9c5cc61094b239377c17
Reviewed-on: https://skia-review.googlesource.com/8606
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>