Use template specialization to generate WindowProperty code

ui/aura/window.h

Index: ui/aura/window.h
diff --git a/ui/aura/window.h b/ui/aura/window.h
index d1cff7b8049994e36da8a15db7dac87b3434b498..79b8467b32630308cfd59b1e0000c355482a3638 100644
--- a/ui/aura/window.h
+++ b/ui/aura/window.h
@@ -52,6 +52,10 @@ class WindowTreeHost;
 template<typename T>
 struct WindowProperty;
 
+namespace subtle {
+class PropertyHelper;
+}
+
 namespace test {
 class WindowTestApi;
 }
@@ -332,7 +336,7 @@ class AURA_EXPORT Window : public ui::LayerDelegate,
   friend class test::WindowTestApi;
   friend class LayoutManager;
   friend class WindowTargeter;
-
+  friend class subtle::PropertyHelper;
   // Called by the public {Set,Get,Clear}Property functions.
   int64 SetPropertyInternal(const void* key,
                             const char* name,