[builtins] Re-add similar String wrapper optimization for StringAdd.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index 1692038f90221e8a195b3cbe31c69f343c2674db..62c1880bcb35c8061dd6f3c4b6ea6b0712127766 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -219,6 +219,7 @@ enum BindingFlags {
   V(NORMALIZED_MAP_CACHE_INDEX, Object, normalized_map_cache)                  \
   V(NUMBER_FUNCTION_INDEX, JSFunction, number_function)                        \
   V(OBJECT_FUNCTION_INDEX, JSFunction, object_function)                        \
+  V(OBJECT_FUNCTION_PROTOTYPE_MAP_INDEX, Map, object_function_prototype_map)   \
   V(OPAQUE_REFERENCE_FUNCTION_INDEX, JSFunction, opaque_reference_function)    \
   V(REGEXP_FUNCTION_INDEX, JSFunction, regexp_function)                        \
   V(REGEXP_RESULT_MAP_INDEX, Map, regexp_result_map)                           \