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The dragging behavior is the same with dragging an app window from the
top of the screen. As it shares some same logic with dragging an app
window, these logic are extracted to TabletModeWindowDragDelegate.

See recorded video:
https://drive.google.com/open?id=1d6t7K-ZuxgpT57b-Mx5gMJ49IjMHX-9X

Bug: 823769
Test: SplitViewTabDraggingTest.OnlyAllowDraggingOnCaptionArea

Change-Id: I09d852ef9eefd1059f2be85fee52e1714b1968fd
Reviewed-on: https://chromium-review.googlesource.com/1150860
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580004}