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The changes in this patch:
. If in split-view mode, the left and right windows remain fixed in the overview
  state.
. Selecting either of the left/right windows returns to the split-view mode with
  the same windows.
. Selecting one of the other windows in the stack gets out of split-view mode,
  and switches to the selected window.

In subsequent patches:
. Allow dragging a window from the stack on top of the left/right windows to
  switch that window and remain in split-view mode.
. Allow dragging one of the left/right windows to get out of split-view mode and
  switch to that window.

BUG=404119
R=oshima@chromium.org

Review URL: https://codereview.chromium.org/495193003

Cr-Commit-Position: refs/heads/master@{#292468}