TabDragging: The dragged window in preview area case.

When dragging a window in overview grid, if the window is dragged into
the preview area, and a left or right semitransparent preview window
shows up indicating the dragged window will be snapped to left or right
side of the screen after releasing the mouse/touch, we should clear the
selection widget on the current selected overview window item and also
clear the ash::kIsDeferredTabDraggingTargetWindowKey property to prevent
the dragged window to merge into the selected overview window item.

See before https://drive.google.com/open?id=14fwUX8yXUoAnJrOklOGDcG2ChZFMpIVJ
and after https://drive.google.com/open?id=1Ps1VXj03UMceMDeZ3_puMSG-PluHAQG2

Bug: 823769
Change-Id: I0cc1a009dc6efc07f3baca5519cbc26e9dbb1ec3
Reviewed-on: https://chromium-review.googlesource.com/1142483
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576927}
8 files changed