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This is mostly the same as mouse support, except:
- 'touchstart' must be triggered first. Then, the subsequent 'touchmove'
  event can start the reorder flow.
- There is no touch equivalent for 'mouseover'. Hence, we emulate it
  by manually checking if we are hovering over another tile using
  using |document.elementsFromPoint|.

Screencast: https://drive.google.com/open?id=1DTbSzrcN9CBzvOWvziHPxz6w7_FIOPqJ

Bug: 851335
Change-Id: I2f91576c86c4d825c310612d4def14fa6fc227f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600597
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665010}