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This allows Nyan to force glFlush() based on the blacklist since
EGL_IMAGE_EXTERNAL_FLUSH_EXT is broken on NVIDIA.

Bug: 833975, 978206
Test: Ran on nyan board to validate no corruption.
Change-Id: Ib229422ec770a401369cf51b882e15c13a34f376
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678381Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672543}