This code uses URLFetcher to fetch chrome:// URLs, which the network service can't do, so this is more than a simple switch to using SimpleURLLoader.

web_ui_url_loader_factory.cc has a URLLoaderFactory that can load chrome:// urls, which we can allow embedders to use (similar to how we do things with FileURLLoaderFactory).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cb02821074ba14de3d6a503ef204b694b25017a4

commit cb02821074ba14de3d6a503ef204b694b25017a4
Author: Matt Menke <mmenke@chromium.org>
Date: Tue Jun 19 15:40:30 2018

Annotate many of the browser_tests disabled under the NetworkService.

Also remove a couple that either no longer exist, have been disabled
generally due to flakiness, or are now passing.

BUG= 844950 ,  844951 ,  844952 ,  853251 ,  844928 ,
BUG= 843205 ,  844949 ,  844925 ,  844939 , 821021,
BUG=853798,  844973 ,  844927 ,  844926 ,  844950 

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I094a012fe2076c7badf86a094140c7d74db183be
Reviewed-on: https://chromium-review.googlesource.com/1104802
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568464}
[modify] https://crrev.com/cb02821074ba14de3d6a503ef204b694b25017a4/testing/buildbot/filters/mojo.fyi.network_browser_tests.filter

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/788008660f92f0e668166c2b3d4c97a8ea2c3e9a

commit 788008660f92f0e668166c2b3d4c97a8ea2c3e9a
Author: John Abd-El-Malek <jam@chromium.org>
Date: Mon Jun 25 20:49:50 2018

Convert WebUIURLFetcher to use SimpleURLLoader so that it can work with the network service.

Bug:  844927 
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Ib0b437f76822800bc393253250ed2660527f276b
Reviewed-on: https://chromium-review.googlesource.com/1110666
Reviewed-by: Xi Han <hanxi@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570169}
[modify] https://crrev.com/788008660f92f0e668166c2b3d4c97a8ea2c3e9a/content/browser/webui/web_ui_url_loader_factory.cc
[modify] https://crrev.com/788008660f92f0e668166c2b3d4c97a8ea2c3e9a/extensions/browser/api/guest_view/web_view/web_view_internal_api.cc
[modify] https://crrev.com/788008660f92f0e668166c2b3d4c97a8ea2c3e9a/extensions/browser/guest_view/web_view/web_ui/web_ui_url_fetcher.cc
[modify] https://crrev.com/788008660f92f0e668166c2b3d4c97a8ea2c3e9a/extensions/browser/guest_view/web_view/web_ui/web_ui_url_fetcher.h
[modify] https://crrev.com/788008660f92f0e668166c2b3d4c97a8ea2c3e9a/extensions/browser/web_ui_user_script_loader.cc
[modify] https://crrev.com/788008660f92f0e668166c2b3d4c97a8ea2c3e9a/extensions/browser/web_ui_user_script_loader.h
[modify] https://crrev.com/788008660f92f0e668166c2b3d4c97a8ea2c3e9a/testing/buildbot/filters/mojo.fyi.network_browser_tests.filter