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This will be used in
URLLoaderFactoryGetter::FlushNetworkInterfaceForTesting to prevent
nested IO message loops.

Hit this in
https://chromium-review.googlesource.com/c/chromium/src/+/1139048.

Bug: 857577
Change-Id: I1b6a53f39737ff0a6d4681b4256991569b91a2d5
Reviewed-on: https://chromium-review.googlesource.com/1142198
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576350}