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- Make fullscreened window can be dragged from the top of the display.

- Keep shelf visible if a window is being dragged from the top of the display.

- Fling the window from the top exceed the velocity threshold should drop the
  window into overview mode.

- Update the window transform during drag.

See recorded video:
https://drive.google.com/file/d/0B5I0jFeLxqIiSnFza0t3bEluUnJ0VVBhUzFiSnNwN0k4eFpv/view?usp=sharing

Bug: 847587
Change-Id: I7f9a423f511c65c083fe11a6deb5847afa4c9a35
Reviewed-on: https://chromium-review.googlesource.com/1134575Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577365}