Allow hidden properties and implement GetIdentityHash

src/objects.h

Index: src/objects.h
===================================================================
--- src/objects.h	(revision 1540)
+++ src/objects.h	(working copy)
@@ -1252,6 +1252,11 @@
   // Return the object's prototype (might be Heap::null_value()).
   inline Object* GetPrototype();
 
+  // Return the object's hidden properties object. If the object has no hidden
+  // properties and create_if_needed is true, then a new hidden property object
+  // will be allocated. Otherwise the Heap::undefined_value is returned.
+  Object* GetHiddenProperties(bool create_if_needed);
+
   // Tells whether the index'th element is present.
   inline bool HasElement(uint32_t index);
   bool HasElementWithReceiver(JSObject* receiver, uint32_t index);