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Fixing crbug.com/847501 and adding two regression tests for it.

Added SchedulerWorker::GetLastUsedTime() to support this fix and re-used
it to suppress the need for the delegate's |is_on_idle_worker_stack_|.

Tweaked WaitForWorkersCleanedUpForTesting() to allow being called
multiple times.

R=fdoray@chromium.org

Bug: 847501
Change-Id: I83f814358f679bdbee49778963ff9d05cd240adc
Reviewed-on: https://chromium-review.googlesource.com/1086978
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565288}