[iOS] Create flag for Settings UI Refresh

This CL creates the flag to be used to change the UI of Settings.

Bug: 922511
Change-Id: Ia94550261f51852620a354e4d1e6282a4013f42e
Reviewed-on: https://chromium-review.googlesource.com/c/1413606
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623651}
5 files changed