splitview: Fix animation not showing up when exiting overview.

[1] changed overview to release overview session and then pass that
object to its observers. Splitview was querying overview controller
for overview session which would be null, so it wouldnt remove the snap
window from overview list, which would cause its animation to be

This cl change splitview to remove the window in OnOverviewEnded and
use the overview session object before snapping.

[1]: https://chromium-review.googlesource.com/c/chromium/src/+/1406169

Test: manual
Bug: 929658
Change-Id: Iabab38034bea49cdafd1a3ad24310298ca8826de
Reviewed-on: https://chromium-review.googlesource.com/c/1459066
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630106}
4 files changed