GitLab

This is an initial CL to allow dragging overview window to snap to
splitview in clamshell mode.

- Add a feature flag "DragToSnapInClamshellMode"
- In clamshell mode, split view mode behaviors are very different with that
in tablet mode. Split view mode is only active during an overview
session. Once the user has selected two windows to snap to both side of
the screen, split view mode is no longer active.

See https://drive.google.com/open?id=15A2CSJxxRfewbPzMvlhe5SmpST1TGJTC
for demo.

Bug: 890029
Change-Id: Iac9cc8f80b28df1d15b761fbb38e30b6a48c3214
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559113
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650538}