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When swiping up from the shelf to drag the active window:
- The active window can be dragged to snap in splitview or be dropped
  in overview. Fling the active window can head to home launcher
  screen if the velocity is large enough.
- If overview is active and splitview is inactive, it's a no-op.
- If overview is active and splitview is active, swiping on the
  snapped window side will drag the snapped window around in overview,
  and swiping on the overview side is a no-op.
- If overview is inactive and splitview is active, swiping an snapped
  window will open overview behind the dragged window. Fling the
  snapped window can only drop the snapped window to overview, but
  can't head to home launcher screen.

Bug: 997885
Change-Id: I55d0e37dd2298c3b4906b53b3ee20038f9e886bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830173
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703949}