GitLab

- Add RunDragLoop and CancelDragLoop to WindowServiceDelegate interface;
- WindowServiceDelegateImpl use ash::DragDropController to provide
  the actual impl;
- WindowTree implements PerformDragDrop and CancelDragDrop via
  the WindowServiceDelegate;

Bug: 837716
Test: WindowServiceDelegateImplTest.*Drag*, WindowTreeTest2.*Drag*
Change-Id: Id5d609d30dd53171b01bfeec60e4547f7a61b4c3
Reviewed-on: https://chromium-review.googlesource.com/1110669
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569418}