This enables more efficient Skia A8 mask blitters.

Change-Id: Ied97707814e4fd893556b2e77a5426187e6e7b26
Reviewed-on: https://chromium-review.googlesource.com/1019461Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552103}