Fix a crash in DesktopNativeWidgetHelperAura when closing tabs because it was dereferncing a delete…

ui/views/widget/desktop_native_widget_helper_aura.cc

Index: ui/views/widget/desktop_native_widget_helper_aura.cc
===================================================================
--- ui/views/widget/desktop_native_widget_helper_aura.cc	(revision 157681)
+++ ui/views/widget/desktop_native_widget_helper_aura.cc	(working copy)
@@ -172,6 +172,11 @@
   return root_window_.get();
 }
 
+void DesktopNativeWidgetHelperAura::OnWindowDestroying(aura::Window* window) {
+  window_->RemoveObserver(this);
+  window_ = NULL;
+}
+
 gfx::Rect DesktopNativeWidgetHelperAura::ModifyAndSetBounds(
     const gfx::Rect& bounds) {
   gfx::Rect out_bounds = bounds;