Virtual Desks 1: Initial scaffolding

This CL adds the very basic scaffolding which will be
used in subsequent CLs to build the virtual desks feature.

Added in this CL:
- The VirtualDesks base::Feature.
- Bar that will later contain desks' thumbnails.
- The "New desk" button, which currently does nothing.
- Moves gesture taps and mouse release overview handling from
  a pre-target handler in WallpaperView to the ShieldView.

Demo: https://bugs.chromium.org/p/chromium/issues/detail?id=866622#c2

BUG=866622

Change-Id: Ie09e7a5b11aaae82fcaf592c009d54ec7245fe10
Reviewed-on: https://chromium-review.googlesource.com/c/1461821
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631513}
22 files changed