In-object double fields unboxing (for 64-bit only).

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 17358e41feb4fc36d26760e0260115277c56b759..f96a9b6e4adfae47fe3322ff5d1a5872c4d5aca8 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -14542,7 +14542,8 @@ RUNTIME_FUNCTION(Runtime_LoadMutableDouble) {
     RUNTIME_ASSERT(field_index.outobject_array_index() <
                    object->properties()->length());
   }
-  Handle<Object> raw_value(object->RawFastPropertyAt(field_index), isolate);
+  Handle<Object> raw_value(
+      JSObject::RawFastBoxedPropertyAt(object, field_index));
   RUNTIME_ASSERT(raw_value->IsMutableHeapNumber());
   return *Object::WrapForRead(isolate, raw_value, Representation::Double());
 }