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This will be used by Skia for async YUV420 readbacks with ES2 ANGLE
contexts.

Bug: 1040643
Change-Id: Ifcc0bff78efcf75764902e8a2910aea7266bf327
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015645Reviewed-by: Jonathan Backer <backer@chromium.org>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: Brian Salomon <bsalomon@google.com>
Cr-Commit-Position: refs/heads/master@{#734632}