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This will let use remove the use of URLRequestContextGetters in the
NetworkService, and also let us adjust URLLoaderFactory lifetime to
so that it lives as long as it has a live URLLoader, so it can own
shared objects.

Bug: 803149
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo;master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I7028886f9df671530ee9863f80dc437adc20cdfb
Reviewed-on: https://chromium-review.googlesource.com/952342Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542188}