GitLab

This CL:
* Adds drag-and-drop behavior to holding space views
* Adds a new base class for holding space browsertests
* Adds browsertests to verify drag-and-drop behavior

Still TODO:
* Add drag image

Bug: 1124581
Change-Id: I4a2e7fe63b26b10b7a7563adcfe7569d095ea7fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2392263
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805094}