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This CL adds checks to fail tests for browsers that do not support longtasks
instead of forcing them to timeout.

Change-Id: I5be54a1ab0a5d003decb58fc1e79380cbce15976
Reviewed-on: https://chromium-review.googlesource.com/1128174Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573900}