Migrate remoting::GcdRestClient to SimpleURLLoader

URLFetcher will stop working with advent of Network Service, and
SimpleURLLoader is the replacement API for most clients.
This CL migrates remoting::GcdRestClient and the respective
unittests away from URLFetcher.

BUG=773295

Change-Id: I177430191cf50c8af3e7424cf9ed2e2b459ea4af
Reviewed-on: https://chromium-review.googlesource.com/1195602
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#587984}
6 files changed