Add missing IDL parser support for 'stringifier readonly attribute'.

tools/idl_parser/idl_parser.py

Index: tools/idl_parser/idl_parser.py
diff --git a/tools/idl_parser/idl_parser.py b/tools/idl_parser/idl_parser.py
index 45f8d7c58276a9d922b21ba6b4cbf2448cd907d4..ff787a46346568708f03dd954ce6a6b326dacd0c 100755
--- a/tools/idl_parser/idl_parser.py
+++ b/tools/idl_parser/idl_parser.py
@@ -554,14 +554,12 @@ class IDLParser(object):
 
   # [36]
   def p_StringifierRest(self, p):
-    """StringifierRest : AttributeRest
+    """StringifierRest : ReadOnly AttributeRest
                        | ReturnType OperationRest
                        | ';'"""
     if len(p) == 3:
       p[2].AddChildren(p[1])
       p[0] = p[2]
-    elif p[1] != ';':
-      p[0] = p[1]
 
   # [37]
   def p_StaticMember(self, p):