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Soon there will be multiple queues with different priorities, so the
trick of posting a task that quits a run loop currentlyl in use in
test_utils.cc will no longer work. In preparation for that this patch
add a better implementation of the same functionality.

Bug: 863341
Change-Id: I643e22a491145a3bae00d06ae9164a450f501298
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1541251Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#646252}