Support WebIDL optional default value syntax

Source/bindings/scripts/idl_definitions.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 457 matching lines @@
 # Arguments
 ################################################################################
 
 class IdlArgument(TypedObject):
     def __init__(self, node):
         self.extended_attributes = {}
         self.idl_type = None
         self.is_optional = node.GetProperty('OPTIONAL')  # syntax: (optional T)
         self.is_variadic = False  # syntax: (T...)
         self.name = node.GetName()
+        self.default_value = None
 
         children = node.GetChildren()
         for child in children:
             child_class = child.GetClass()
             if child_class == 'Type':
                 self.idl_type = type_node_to_type(child)
             elif child_class == 'ExtAttributes':
                 self.extended_attributes = ext_attributes_node_to_extended_attributes(child)
             elif child_class == 'Argument':
                 child_name = child.GetName()
                 if child_name != '...':
                     raise ValueError('Unrecognized Argument node; expected "...", got "%s"' % child_name)
                 self.is_variadic = child.GetProperty('ELLIPSIS') or False
+            elif child_class == 'Default':
+                self.default_value = child.GetName()
             else:
                 raise ValueError('Unrecognized node class: %s' % child_class)
 
 
 def arguments_node_to_arguments(node):
     # [Constructor] and [CustomConstructor] without arguments (the bare form)
     # have None instead of an arguments node, but have the same meaning as using
     # an empty argument list, [Constructor()], so special-case this.
     # http://www.w3.org/TR/WebIDL/#Constructor
     if node is None:
@@ skipping 188 matching lines @@
     child_class = child.GetClass()
     if child_class != 'Type':
         raise ValueError('Unrecognized node class: %s' % child_class)
     return type_node_to_type(child)
 
 
 def union_type_node_to_idl_union_type(node, is_nullable=False):
     member_types = [type_node_to_type(member_type_node)
                     for member_type_node in node.GetChildren()]
     return IdlUnionType(member_types, is_nullable=is_nullable)