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Adds an entry to about flags page in iOS in order to control the feature.

Bug: 841758
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Id61d8f4a63f0c6a89ec4c643640e1870d58c2ae7
Reviewed-on: https://chromium-review.googlesource.com/1055904
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558026}