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This CL improves tests in external/wpt/longtask-timing by:
* Increasing the length of a longtask from ~51 to ~60 to prevent a
coarse performance.now() granularity from affecting the tests.
* Wrapping the long tasks or iframes containing longtasks inside onload
handlers to ensure that the PerformanceObserver is registered by the
time the longtask is executed.

Test: third_party/blink/tools/run_web_tests external/wpt/longtask-timing
--iterations=1000 did not produce flakes.

Bug: 796668
Change-Id: Ic2ce59f7d221566f5443da9e9c7d36c7bd7bbe6f
Reviewed-on: https://chromium-review.googlesource.com/1138725
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575396}