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

src/string-stream.cc

Index: src/string-stream.cc
diff --git a/src/string-stream.cc b/src/string-stream.cc
index 196e5bb4a16fac72b43118de8f9f9c6f4754e78b..98ef2e81889c1d978bebccb151bb8d44460a4a0c 100644
--- a/src/string-stream.cc
+++ b/src/string-stream.cc
@@ -343,8 +343,13 @@ void StringStream::PrintUsingMap(JSObject* js_object) {
         }
         Add(": ");
         FieldIndex index = FieldIndex::ForDescriptor(map, i);
-        Object* value = js_object->RawFastPropertyAt(index);
-        Add("%o\n", value);
+        if (map->IsUnboxedDoubleField(index)) {
+          Add("<unboxed double> %.16g\n",
+              js_object->RawFastDoublePropertyAt(index));
+        } else {
+          Object* value = js_object->RawFastPropertyAt(index);
+          Add("%o\n", value);
+        }
       }
     }
   }