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When using SkiaRenderer on Windows use the Skia readback APIs instead of
GLRendererCopier by default.

The Skia readback API doesn't support RGBA_TEXTURE readback format. This
is only used on Chrome OS in production but some cc_unittests exercise
it. Don't run those tests on Windows with SkiaRenderer. We still
exercise the same test with RGBA_BITMAP which is supported.

Bug: 1042646, 1046788
Change-Id: Ida4a1a921c838ee7a2d628da4535d9367140ff04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2019466
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736578}