Add support for EXT_image_flush_external extension

BUG=chromium:478339
TEST=Boot to UI

Signed-off-by: Vince Hsu <vince.h@nvidia.com>
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2109803003
Cr-Original-Commit-Position: refs/heads/master@{#405659}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8baf5bd7df5891470b8dfe4c6de7998bd60170b1
2 files changed