GitLab

This CL adds the initial drag image that is displayed when dragging
items out of Holding Space UI. Follow up CLs will:

- Render screenshot items differently
- Render at most two items with an overflow badge if necessary

Note that this CL does not use ui::Layer's to accomplish the desired
drop shadow effect because a solution could not be identified to paint
a view hierarchy with its layer tree to an SkBitmap.

Demo: http://shortn/_mWLGXsTcTY
Note that the screen recorder did not capture the cursor moving with
the drag image but it actually does so on device.

Bug: 1139113
Change-Id: I569bf42b31beb19459f85a80bfc1c3f673a40b1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476728
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818024}