Update V8 to r3121 as required for WebKit update.

Change-Id: Ic53e0aef9a9eb9b71ee7d25a8aef61520bba899c
diff --git a/src/memory.h b/src/memory.h
index c64699e..503492a 100644
--- a/src/memory.h
+++ b/src/memory.h
@@ -63,6 +63,10 @@
   static Object*& Object_at(Address addr)  {
     return *reinterpret_cast<Object**>(addr);
   }
+
+  static Handle<Object>& Object_Handle_at(Address addr)  {
+    return *reinterpret_cast<Handle<Object>*>(addr);
+  }
 };
 
 } }  // namespace v8::internal