[iOS][MF] Create flag for manual fallback phase 2

Bug: 845472
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ic04a26dad55ac2125b6c3c99737fb59fa21d2496
Reviewed-on: https://chromium-review.googlesource.com/1221310
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590693}
5 files changed