Fix performance regression introduced in r28961

src/runtime/runtime-function.cc

Index: src/runtime/runtime-function.cc
diff --git a/src/runtime/runtime-function.cc b/src/runtime/runtime-function.cc
index d644d37e42b2fff45685eb2c7604c5c68c801a3d..89dd72c074e49fcdbdac211da6b3f38236a56f43 100644
--- a/src/runtime/runtime-function.cc
+++ b/src/runtime/runtime-function.cc
@@ -447,6 +447,9 @@ RUNTIME_FUNCTION(Runtime_FunctionBindArguments) {
       isolate->native_context()->bound_function_map());
   JSObject::MigrateToMap(bound_function, bound_function_map);
   Handle<String> length_string = isolate->factory()->length_string();
+  // These attributes must be kept in sync with how the bootstrapper
+  // configures the bound_function_map retrieved above.
+  // We use ...IgnoreAttributes() here because of length's read-onliness.
   PropertyAttributes attr =
       static_cast<PropertyAttributes>(DONT_ENUM | READ_ONLY);
   RETURN_FAILURE_ON_EXCEPTION(