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There are 3 cases in clamshell to tablet mode transition:
1) overview is active but splitview is not: keep overview active during
   transition.
2) overview and splitview are both active: keep them both active during
   transition.
3) overview and splitview are both inactive: keep the current behavior.
   a) if the top window is a snapped window, put it in splitview
   b) if the second top window is also a snapped window that snaps to the
      opposite side of the screen, put it in splitview as well
   c) if the top window is not a snapped window, maximize or fullscreen
      all windows when entering tablet mode.

There are 4 cases in tablet to clamshell mode transition:
1) overview is active but splitview is not: keep overview active during
   transition.
2) overview and splitview are both active: keep them both active during
   transition.
3) overview is inactive but splitview is active (i.e., two snapped windows),
   splitview will be no longer active after transition. But the two snapped
   window will still keep snapped in clamshell mode.
4) overview and splitview are both inactive: keep the current behavior,
   i.e., restore all windows to its old window state before entering tablet
   mode.


Bug: 890029
Change-Id: I7769021f74c3e9d591b19258be2821992d780451
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1658955
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670678}