Port GCM's {Checkin, Unregistration}Request to SimpleURLLoader

URLFetcher will stop working with advent of Network Service, and
SimpleURLLoader is the replacement API for most clients.

This also removes the no-longer needed URLRequestContext-based methods
in GCMRequestTestBase, and fixes the regression of CompleteFetch
overrides being bypassed in the previous RegistrationRequest port.

Bug: 844986, 844988

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Icdb2e768b274618ce5c6d1fceda6ae9b76c4c275
Reviewed-on: https://chromium-review.googlesource.com/1135291
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575351}
15 files changed