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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
Change-Id: I035c087550a2d9a67cda1aed88553c16967d04b6
Reviewed-on: https://chromium-review.googlesource.com/c/1349451
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610669}