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This performs a task flush similar to TaskScheduler::FlushForTesting()
except it is async. It calls the callback if TaskScheduler::Flush()
would have returned.

BUG=804930

Change-Id: I533ab5a2fbaccd79a220aad3a535fa37ed7df823
Reviewed-on: https://chromium-review.googlesource.com/883607
Commit-Queue: Robert Liao <robliao@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532258}