tablet: Adjust what happens when you drag a snapped window from the top.

BEFORE:
1. In split view, when you drag a window from the top, it will snap to
one side or the other, unless you fling toward the opposite side or you
drag onto the drop target.
2. In split view, the preview area never appears while you drag a window
from the top.

AFTER:
1. In split view, consistent with when split view is not active, you can
drag a window from the top and drop it into overview.
2. The preview area behavior (including animations) in split view is
consistent between when you drag a window from the top and when you drag
a window from overview. The drop target (which only appears when you
drag a window from the top, as Issue 940710 has not yet been addressed)
is not shown when the preview area is shown; this behavior is consistent
with when split view is not active.

Test: ash_unittests SplitView*Test.*
Bug: 878294
Bug: 940708
Change-Id: I5294982155b169e385b80f526c2abbabea4712ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1518414
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642183}
5 files changed