Add user action for changing the active window in Ash overview mode

ash/wm/overview/window_selector.h

Index: ash/wm/overview/window_selector.h
diff --git a/ash/wm/overview/window_selector.h b/ash/wm/overview/window_selector.h
index 254d9dc334e3501d67600811a1c72be78267f2f1..7c783b8cc3412f6ccffa5ebe44463470f522c92e 100644
--- a/ash/wm/overview/window_selector.h
+++ b/ash/wm/overview/window_selector.h
@@ -84,6 +84,9 @@ class ASH_EXPORT WindowSelector
   // Called when the last window selector item from a grid is deleted.
   void OnGridEmpty(WindowGrid* grid);
 
+  // Activates |window|.
+  void SelectWindow(aura::Window* window);
+
   bool restoring_minimized_windows() const {
     return restoring_minimized_windows_;
   }