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This fixes an issue that FlushDone() in VideoEncoderClient will
never be called when the test runs with --disable_validator and
the VideoEncodeAccelerator doesn't support Flush().

Bug: None
Test: video_encode_acclerator_tests --disable_validator on trogdor
Change-Id: I8701a971deaa4cacfcd55a2c708de437a79ad863
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537507Reviewed-by: David Staessens <dstaessens@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828005}