GitLab

Correct V4L2VEA to execute flush callback by GPU main child thread.

BUG=b:71882314
TEST=video_encode_accelerator_unittest

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;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: If6eb0c8f87dcd59a0006c811514f230e8a023f0e
Reviewed-on: https://chromium-review.googlesource.com/989873Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548016}