Add section in docs for running WPT tests

Bug: 747099
Change-Id: I27de4ab887dd0b9479e6ad87383a9446b8951915
Reviewed-on: https://chromium-review.googlesource.com/752145
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Jeff Carpenter <jeffcarp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513938}
1 file changed