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This can be used to make sure that tasks are being flushed
through a given task runner at a desired rate, a signal that can
be used to determine if tasks are blocking a thread.

BUG=Internal b/23908801
BUG=Internal b/36699101
TEST=cast_base_unittests

Change-Id: I22dc48134ac46b2bd00bccee8105a7580d1072c9
Reviewed-on: https://chromium-review.googlesource.com/664086
Commit-Queue: Simeon Anfinrud <sanfin@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501643}