[ios] Migrate URLDownloader to SimpleURLLoader

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

BUG=773295

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I8abfd9c9d9e66757c555479fe6a153e3dc08b9c5
Reviewed-on: https://chromium-review.googlesource.com/1234635
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#592808}
6 files changed