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Fixes overlap with https://chromium-review.googlesource.com/767375

Added unit test this time.

This is a preliminary step for the subsequent CL
https://chromium-review.googlesource.com/c/chromium/src/+/766787

Bug: 785201, 788123
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: I70620697109fdef7d036be4c26fcecb763ddb4ba
Tests: media_unittests --gtest_filter=*Vaapi*SupportedPlatforms*
Reviewed-on: https://chromium-review.googlesource.com/788051Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Julien Isorce <julien.isorce@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519657}