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This Cl updates the usage documentation for the video_encode_accelerator_tests
and video_encode_accelerator_perf_tests with the new "--codec" parameter.

TEST=gitiles

BUG=None

Change-Id: I1bfed4af765f2b4ac8ce4323f2209c29e710a382
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215656
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773568}