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Originally, VEA calls the flush callback before notifying the client
that the last frame buffer is ready. This CL changed the calling
order to ensure all frames buffer are sent back to the client before
VEA notifies the flush is completed.

BUG=b:71882314
TEST=run video_encode_accelerator_unittest at device

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I235411734dff3200df3b36b1f14211fbd0d41c8f
Reviewed-on: https://chromium-review.googlesource.com/956653Reviewed-by: Pawel Osciak <posciak@chromium.org>
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542059}