chromeos: makes it possible for windows to outlive WindowTreeClient

ui/aura/window.h

Index: ui/aura/window.h
diff --git a/ui/aura/window.h b/ui/aura/window.h
index 1ab83f04b9eea8cd9ce3a7df9653fc99b3d101c0..929a1f8144d01f0fa0f850e1e91853a64c2aa974 100644
--- a/ui/aura/window.h
+++ b/ui/aura/window.h
@@ -51,8 +51,9 @@ namespace aura {
 
 class LayoutManager;
 class WindowDelegate;
-class WindowPort;
 class WindowObserver;
+class WindowPort;
+class WindowPortForShutdown;
 class WindowTreeHost;
 
 // Defined in class_property.h (which we do not include)
@@ -329,6 +330,7 @@ class AURA_EXPORT Window : public ui::LayerDelegate,
   friend class LayoutManager;
   friend class PropertyConverter;
   friend class WindowPort;
+  friend class WindowPortForShutdown;
   friend class WindowTargeter;
   friend class test::WindowTestApi;
 
@@ -440,6 +442,9 @@ class AURA_EXPORT Window : public ui::LayerDelegate,
   // pointer in |port_| so that it can still be accessed during destruction.
   // This is important as deleting the WindowPort may result in trying to lookup
   // the WindowPort associated with the Window.
+  //
+  // NOTE: this value is reset for windows that exist when WindowTreeClient
+  // is deleted.
   std::unique_ptr<WindowPort> port_owner_;
   WindowPort* port_;