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This CL moves Event Timing tests to web-platform-tests. Relevant changes:
* Move to using test_driver.click.
* Replace setTimeout with step_timeout.
* Move to using a python-based slow image resource.

Bug: 841224
Change-Id: I21f322546815d7d6bcec4dd1703de50a9296d992
Reviewed-on: https://chromium-review.googlesource.com/c/1348855
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610504}