Port GCM's RegistrationRequest to SimpleURLLoader

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

(Other request types will follow; they were not done for now to keep CL size down, since there is a
 lot of wiring of pointers through and test infra stuff).

Bug: 844987
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;luci.chromium.try:linux_mojo;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I1dc5efda6d239509d060f154cedf57ecf73edff2
Reviewed-on: https://chromium-review.googlesource.com/1110705
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574468}
20 files changed