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Use base::FeatureList and about:flags to enable V2 feature.
Default for this flag is DISABLED.

Bug: 657076
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I4b5ee8c95a79f533fded6b717efa0dd6c0bae895
Reviewed-on: https://chromium-review.googlesource.com/900543Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537860}