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This CL adds additional documentation to the video_decode_accelerator_tests
command, describing supported parameters such as --vmodule and --metadata.

TEST=ran new VDA tests on nocturne

BUG=None

Change-Id: Ic5a54ca404a41a6f31a47d29640a4683f83ffa8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1558735
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649742}