IDL: Support optional union type arguments with default values

Source/bindings/scripts/v8_union.py

Index: Source/bindings/scripts/v8_union.py
diff --git a/Source/bindings/scripts/v8_union.py b/Source/bindings/scripts/v8_union.py
index 791e95ea7d0c650c598a6e126cb6c562a17189a5..9d4fae6132ef654bde08bbfb2c4e7a71f452d2fb 100644
--- a/Source/bindings/scripts/v8_union.py
+++ b/Source/bindings/scripts/v8_union.py
@@ -74,17 +74,11 @@ def container_context(union_type, interfaces_info):
             if dictionary_type:
                 raise Exception('%s is ambiguous.' % union_type.name)
             dictionary_type = context
-        elif member.base_type == 'boolean':
-            if boolean_type:
-                raise Exception('%s is ambiguous.' % union_type.name)
+        elif member is union_type.boolean_member_type:
             boolean_type = context
-        elif member.is_numeric_type:
-            if numeric_type:
-                raise Exception('%s is ambiguous.' % union_type.name)
+        elif member is union_type.numeric_member_type:
             numeric_type = context
-        elif member.is_string_type:
-            if string_type:
-                raise Exception('%s is ambiguous.' % union_type.name)
+        elif member is union_type.string_member_type:
             string_type = context
         else:
             raise Exception('%s is not supported as an union member.' % member.name)