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This CL adds documentation with example usages for the test runner.

Bug: 1104318
Change-Id: Ibae86ebe4dd893b5eef01db5699d6310fad4cb53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315478
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791143}