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This CL replaces the existing blocking call to SkImage::readPixels() with
GLHelper::ReadbackTextureAsync() and GLHelper::CreateReadbackPipelineYUV()
callbacks for texture backed images.
In order to use for these calls, a GLHelper instance is lazily created and
owned by blink::WebGraphicsContext3DProvider.

Bug: 727385
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Iad8561183afddff0794038213ee22cec3e04ed76
Reviewed-on: https://chromium-review.googlesource.com/756474
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Justin Novosad <junov@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516979}