And configure it to 50/50 on Canary and Dev.

I'll submit the finch config after this lands.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/acb8f7c6733e5add17b502dd11088be5eb3f7567

commit acb8f7c6733e5add17b502dd11088be5eb3f7567
Author: spqchan <spqchan@chromium.org>
Date: Thu Aug 03 15:38:56 2017

[Mac] Feature Flag for the Fullscreen Toolbar Reveal Animation

Create a feature flag for this feature.

Bug:  751784 
Change-Id: I49ba9af40f68496b1e59f3e5f9fc5e0fa068ae6f
Reviewed-on: https://chromium-review.googlesource.com/598628
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Sarah Chan <spqchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491746}
[modify] https://crrev.com/acb8f7c6733e5add17b502dd11088be5eb3f7567/chrome/browser/about_flags.cc
[modify] https://crrev.com/acb8f7c6733e5add17b502dd11088be5eb3f7567/chrome/browser/ui/cocoa/fullscreen/fullscreen_toolbar_controller.mm
[modify] https://crrev.com/acb8f7c6733e5add17b502dd11088be5eb3f7567/chrome/common/chrome_features.cc
[modify] https://crrev.com/acb8f7c6733e5add17b502dd11088be5eb3f7567/chrome/common/chrome_features.h
[modify] https://crrev.com/acb8f7c6733e5add17b502dd11088be5eb3f7567/tools/metrics/histograms/enums.xml