Enable MaskAsBlending cc pixeltests on Vulkan

Following a fix to SwiftShader Vulkan's handling of BGRA textures, these
tests are now passing for Skia Vulkan, so enable them. The fuzziness
numbers can also be decreased a little bit.

The fuzziness definitions for anti-aliasing and for non-software are
very similar, so merge them.

Bug: 963446
Change-Id: I9dd34702c16e20c46f1550169ab35a8779bad3f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1691085
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Sean Gilhuly <sgilhuly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679629}