Enable graphite in pixel tests

Enable SkiaRenderer on Graphite/Dawn for cc_unittests and viz_unittests.
Both Linux and Windows will use Dawn/Vulkan on swiftshader so should
work fine. The tests will run with asan/msan/tsan on Linux which gives
us extra coverage missing on other platforms.

Also remove the enable_skia_graphite_gtests GN arg since it was only
used by enable_skia_graphite_tests buildflag. It doesn't seem necessary
to split the logic across two separate GN files.

Bug: none
Change-Id: If186c5cd017f7119fed52136bd2db8edd66c0b60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6043495
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Kyle Charbonneau <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1387843}