GitLab

- Add TestDragDropClient based on ash::DragDropController to map mouse
  events to DragTargetEvents and dispatch them;
- Implement RunDragLoop/CancelDragLoop in test_ws using
  TestDragDropClient;
- Re-enable RunDragLoop/CancelDragLoop;

Bug: 864616
Change-Id: I349e5c80abe8f20e27a5da34992c15cc611a98b3
Reviewed-on: https://chromium-review.googlesource.com/1152461
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578690}