GitLab

- make it a collection of statics instead of a singleton object
- move to //ash/public
- use it in BrowserNonClientFrameViewAsh to match
  ash::WorkspaceEventHandler

Bug: none
Change-Id: I64f290e63f6f729239b52a67f84c39e62f04c4f1
Reviewed-on: https://chromium-review.googlesource.com/c/1406301Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622129}