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Originally, we need to add "--disable_flush" argument when running
the VEA unittest if the encoder does not support flush. This CL added
a method IsFlushSupported() to check if the encoder support flush.

BUG=820313
TEST=Run vea unittest at eve and nyan_big without
     "--disable_flush" argument

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Id6a59546b0a13df15ca4ff2825eb70c848eaf704
Reviewed-on: https://chromium-review.googlesource.com/1170671Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588457}