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Currently VizProcessContextProvider assumes no alpha. Android needs
alpha in cases where it is rendering on top of a video overlay. Plumb
the alpha request through via RendererSettings.

Bug: 869131
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I6c51a07b141e61d95b2e25183156ebf15daeca38
Reviewed-on: https://chromium-review.googlesource.com/1155715Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580015}