Make GaiaAuthFetcher use the network::SimpleURLLoader

As part of the effort to move to the network service, changing the
GaiaAuthFetcher to use network::SimpleURLLoader instead of using
net::URLRequest.

This changes the API to take in a SharedURLLoaderFactory instead of a
net::URLRequestContextGetter and affects many callers.

Many tests also had to be changed, as it is common to mock a URLRequest
and this is now done using a TestURLLoaderFactory.

Bug: 840396
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;luci.chromium.try:linux_mojo;master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ib93075b06855a0a5ba9349ae64e2ad43b02e0c6f
Reviewed-on: https://chromium-review.googlesource.com/1091533
Commit-Queue: Jay Civelli <jcivelli@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575332}
131 files changed