Should be a handful of trivial layout test changes.

By adding and using the srcover_bgra_8888, one of the cc unit test golden images
needs to change just a little bit (literally, one bit of one pixel as far as I 
can tell).  This new fused stage does the srcover blend math in [0,255] bias 
instead of [0,1], so the rounding can change ever so slightly.

Change-Id: I5e2d526496b030d06e5820dd9f3dd56a1fc571f6
Reviewed-on: https://chromium-review.googlesource.com/737859


Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512887}