Support WebIDL optional default value syntax

Source/bindings/scripts/idl_definitions.py

Index: Source/bindings/scripts/idl_definitions.py
diff --git a/Source/bindings/scripts/idl_definitions.py b/Source/bindings/scripts/idl_definitions.py
index 2230c8f6e207153d8145a7d7dca1560077f1bf7b..2ad15094f794e36a39e3961cac95bb5aaf489e91 100644
--- a/Source/bindings/scripts/idl_definitions.py
+++ b/Source/bindings/scripts/idl_definitions.py
@@ -475,6 +475,7 @@ class IdlArgument(TypedObject):
         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:
@@ -488,6 +489,8 @@ class IdlArgument(TypedObject):
                 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)