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This gives the child process time to flush I/O. This can help avoid the OS restarting in the middle
of flushing data. It's also needed because swarming jobs fail when the browser quits with child
processes still alive.

An example flake is
https://chromium-swarm.appspot.com/task?id=3d8cd8e056a4b310&refresh=10&show_raw=1. Roughly half of
the layout test runs on Win Mojo are flaking as a result.

Bug: 820996
Change-Id: I9c3b13f83c716f020f555cbe2aa14e20c8825ed3
Reviewed-on: https://chromium-review.googlesource.com/1066622Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560368}