Move Ash caption buttons to //ash/public/cpp

1. Remove AshFrameSizeButton dependency on PhantomWindowController
   via a new interface. For Mash Chrome browser frames, this will
   be implemented via Mojo (for now, that's a TODO).

2. Add ash::TabletMode utility to //ash/public/cpp, which allows
   code in //ash/public/cpp to get the status of tablet mode from
   any process (each process has to provide a callback).

3. Move the files from private Ash to the new directory.

Bug: 887051

Change-Id: Ib790a89fae89f02b92b36e1b165d59c9e28b59f7
Reviewed-on: https://chromium-review.googlesource.com/1228978
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593336}
53 files changed