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This CL adds a new histogram entry that tells us which gesture
navigation UI (among Chrome's own gesture and OS-provided one)
is used. It is logged on Chrome startup in the devices that has
the Chrome gesture UI is enabled to get an idea on how many devices
have OS gesture vs. Chrome's own UI.

Bug: 1128629
Change-Id: I8062aa8372a1db9050345fb9bb79e10920cb8a2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2448713
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Reviewed-by: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817747}