IDL: Use IdlNullableType wrapper to represent nullable types

Source/bindings/scripts/v8_dictionary.py

Index: Source/bindings/scripts/v8_dictionary.py
diff --git a/Source/bindings/scripts/v8_dictionary.py b/Source/bindings/scripts/v8_dictionary.py
index 6dc772fdcf99944f5204b50dc7bcc8302e07093a..53892bc917ef282e972ed0e5325e36f76f6332d4 100644
--- a/Source/bindings/scripts/v8_dictionary.py
+++ b/Source/bindings/scripts/v8_dictionary.py
@@ -52,12 +52,9 @@ def member_context(member):
     idl_type.add_includes_for_type()
 
     def idl_type_for_default_value():
-        copied_type = copy.copy(idl_type)
-        # IdlType for default values shouldn't be nullable. Otherwise,
-        # it will generate meaningless expression like
-        # 'String("default value").isNull() ? ...'.
-        copied_type.is_nullable = False
-        return copied_type
+        if idl_type.is_nullable:
+            return idl_type.inner_type
+        return idl_type
 
     def default_values():
         if not member.default_value: