Also if one of the 3 windows is not snappable, and a window is snapped. The "App does not support split-screen" does not appear until that window is attempted to dragged.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/306c040b2263619e7efe15fa66226b0551525244

commit 306c040b2263619e7efe15fa66226b0551525244
Author: Sammie Quon <sammiequon@google.com>
Date: Wed Feb 14 02:11:48 2018

overview: Minimized window items not stacked properly.

Window items which are based on minimized windows have some issues.
The "App does not support split-screen" label does not show up, and
dragged windows are not always on top of the rest of the window items.

The current logic always stacks the window item widget on top of the
original window. But the original window is actually below the minimized
widget on z order, so the item widget ends up also being below. The
minimized widget needs to be stacked below, or the item widget needs to
be stacked above the minimized widget. This CL attempts the latter.

Also cache the command line flag for new overview ui, to reduce the
amount of calls.

Also center the "App does not support split-screen" label in the window
selector item minus the title bar.

Test: manual
Bug:  810461 
Change-Id: I1bd10fe0d206b175112d33a359775c13a5b9e19e
Reviewed-on: https://chromium-review.googlesource.com/909301
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima (In Tokyo) <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536608}
[modify] https://crrev.com/306c040b2263619e7efe15fa66226b0551525244/ash/wm/overview/overview_utils.cc
[modify] https://crrev.com/306c040b2263619e7efe15fa66226b0551525244/ash/wm/overview/overview_utils.h
[modify] https://crrev.com/306c040b2263619e7efe15fa66226b0551525244/ash/wm/overview/scoped_transform_overview_window.h
[modify] https://crrev.com/306c040b2263619e7efe15fa66226b0551525244/ash/wm/overview/window_selector_item.cc
[modify] https://crrev.com/306c040b2263619e7efe15fa66226b0551525244/ash/wm/overview/window_selector_item.h
[modify] https://crrev.com/306c040b2263619e7efe15fa66226b0551525244/ash/wm/overview/window_selector_unittest.cc