ash: Make wm::WindowState's c'tor private.

ash/wm/window_state.h

Index: ash/wm/window_state.h
diff --git a/ash/wm/window_state.h b/ash/wm/window_state.h
index 936caaa18a115b2041c839d5d272627142aa726d..602c4bf17575a703e03274daa6958350e1ed59cf 100644
--- a/ash/wm/window_state.h
+++ b/ash/wm/window_state.h
@@ -77,7 +77,7 @@ class ASH_EXPORT WindowState : public aura::WindowObserver {
     DISALLOW_COPY_AND_ASSIGN(State);
   };
 
-  explicit WindowState(aura::Window* window);
+  // Call GetWindowState() to instantiate this class.
   virtual ~WindowState();
 
   aura::Window* window() { return window_; }
@@ -304,8 +304,11 @@ class ASH_EXPORT WindowState : public aura::WindowObserver {
  private:
   friend class DefaultState;
   friend class ash::MaximizeModeWindowState;
+  friend ASH_EXPORT WindowState* GetWindowState(aura::Window*);
   FRIEND_TEST_ALL_PREFIXES(WindowAnimationsTest, CrossFadeToBounds);
 
+  explicit WindowState(aura::Window* window);
+
   WindowStateDelegate* delegate() { return delegate_.get(); }
 
   // Returns the window's current show state.