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This CL moves the tests in http/tests/event-timing to external/wpt/event-timing.
The slow image is change from php based to python based. The click is now
handled by test driver. and setTimeout is replaced with step_timeout.

Bug: 841224, 908187, 907948, 907949
Change-Id: Ia6359878666cfb96645aa3b7cd7736ce9df97683
Reviewed-on: https://chromium-review.googlesource.com/c/1352617Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612214}