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This CL adds documentation to the video_decode_accelerator_perf_tests command,
describing how to run the tests.

TEST=ran new VDA tests on nocturne

BUG=925714

Change-Id: I8c06e9f74dea9a0462cb1b1d68de2aea2ff12d6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1553128
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649738}