Thread-safety comment for bug 479198. r=brendan.

js/src/jsinterp.cpp

@@ -297,6 +297,12 @@ js_FillPropertyCache(JSContext *cx, JSObject *obj, jsuword kshape,
             /*
              * Make sure that a later shadowing assignment will enter
              * PurgeProtoChain and invalidate this entry, bug 479198.
+             *
+             * This is thread-safe even though obj is not locked. Only the
+             * DELEGATE bit of obj->classword can change at runtime, given that
+             * obj is native; and the bit is only set, never cleared. And on
+             * platforms where another CPU can fail to see this write, it's OK
+             * because the property cache and JIT cache are thread-local.
              */
             OBJ_SET_DELEGATE(cx, obj);
         }