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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
TBR=xiyuan@chromium.org

Reviewed-on: https://chromium-review.googlesource.com/c/1274197
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#598827}
Change-Id: I3e7377c053d14fcf3fd882abff2f860a88a61332
Reviewed-on: https://chromium-review.googlesource.com/c/1280674Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599693}