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This CL wires all the parts and enables Manual Fallback, still work in
progress, behind it's flag.

Bug: 845472
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I8bef297135511a1d122dcac0c6199e263f112566
Reviewed-on: https://chromium-review.googlesource.com/1203994
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590323}