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A valid use case in a browser test was recently presented to me. And
while I ultimately plan to support this in ScopedTaskEnvironment as
well, it isn't for now.

R=dpranke@chromium.org

Bug: None
Change-Id: Id4197c8a8cd567c5daaf4b1b39fe8f34fb8b3549
Reviewed-on: https://chromium-review.googlesource.com/1027809Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553736}