Make sure WindowObservers are removed from window before destruction

ash/wm/window_state.cc

Index: ash/wm/window_state.cc
diff --git a/ash/wm/window_state.cc b/ash/wm/window_state.cc
index 071307fd5437e36a1f5c2c2ded4ec3d2bb919b24..0a47384717531ce788a517b17119d15ac6506220 100644
--- a/ash/wm/window_state.cc
+++ b/ash/wm/window_state.cc
@@ -270,10 +270,6 @@ void WindowState::OnWindowPropertyChanged(aura::Window* window,
   }
 }
 
-void WindowState::OnWindowDestroying(aura::Window* window) {
-  window_->RemoveObserver(this);
-}
-
 void WindowState::SnapWindow(WindowShowType left_or_right,
                              const gfx::Rect& bounds) {
   // Compute the bounds that the window will restore to. If the window does not