[iOS] Use URL loader factory

This replaces the old URLRequestContextGetter, and is 1/2 CLs to fix
the internal repo.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ia69ee02311aa9923156f9f56961fd3c315fb0967
Reviewed-on: https://chromium-review.googlesource.com/1240122
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Eric Noyau <noyau@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593520}
6 files changed