athena: Allow getting into split-view mode from overview-mode.

athena/wm/split_view_controller.h

Index: athena/wm/split_view_controller.h
diff --git a/athena/wm/split_view_controller.h b/athena/wm/split_view_controller.h
index 337b75b2471a4ec73e036225764b6e3fb78e9af4..d7dc20c0328f821e755a97051add5040b56fdf7b 100644
--- a/athena/wm/split_view_controller.h
+++ b/athena/wm/split_view_controller.h
@@ -33,6 +33,11 @@ class SplitViewController : public BezelController::ScrollDelegate,
 
   bool IsSplitViewModeActive() const;
 
+  // Activates split-view mode with |left| and |right| windows. If |left| and/or
+  // |right| is NULL, then the first window in the window-list (which is neither
+  // |left| nor |right|) is selected instead.
+  void ActivateSplitMode(aura::Window* left, aura::Window* right);
+
  private:
   enum State {
     // Split View mode is not active. |left_window_| and |right_window| are