splitscreen: Adjust bubble dialog bounds if splitscreen is active.

When a dialog is added to a window that's currently showig in
splitscreen, we should adjust the dialog bounds to show the dialog
aligned within its own side of the split, rather than across the divider
bar.

Note the divider is still placed on top of the snapped windows and its
transient children if they are placed in default container.

Bug: 878268
Change-Id: Ibdd71f1d42aac419ef73ae9f7a2f78af6bf36880
Reviewed-on: https://chromium-review.googlesource.com/c/1274197
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598827}
3 files changed