IDL: Support argument default values

Source/bindings/templates/methods.cpp

 {##############################################################################}
 {% macro generate_method(method, world_suffix) %}
 {% filter conditional(method.conditional_string) %}
 static void {{method.name}}{{method.overload_index}}Method{{world_suffix}}(const v8::FunctionCallbackInfo<v8::Value>& info)
 {
     {# Local variables #}
     {% if method.has_exception_state %}
     ExceptionState exceptionState(ExceptionState::ExecutionContext, "{{method.name}}", "{{interface_name}}", info.Holder(), info.GetIsolate());
     {% endif %}
     {# Overloaded methods have length checked during overload resolution #}
@@ skipping 66 matching lines @@
 {% macro generate_arguments(method, world_suffix) %}
 {% for argument in method.arguments %}
 {{generate_argument_var_declaration(argument)}};
 {% endfor %}
 {
     {% if method.arguments_need_try_catch %}
     v8::TryCatch block;
     V8RethrowTryCatchScope rethrow(block);
     {% endif %}
     {% for argument in method.arguments %}
+    {% if argument.default_value %}
+    if (info.Length() > {{argument.index}}) {
+        {{generate_argument(method, argument, world_suffix) | indent(8)}}
+    } else {
+        {{argument.name}} = {{argument.default_value}};
+    }
+    {% else %}
     {{generate_argument(method, argument, world_suffix) | indent}}
+    {% endif %}
     {% endfor %}
 }
 {% endmacro %}
 
 
 {######################################}
 {% macro generate_argument_var_declaration(argument) %}
 {% if argument.is_callback_interface %}
 {# FIXME: remove EventListener special case #}
 {% if argument.idl_type == 'EventListener' %}
@@ skipping 458 matching lines @@
     {% endif %}
     {{constructor.cpp_type}} impl = {{cpp_class}}::createForJSConstructor({{constructor.argument_list | join(', ')}});
     {% if is_constructor_raises_exception %}
     if (exceptionState.throwIfNeeded())
         return;
     {% endif %}
 
     {{generate_constructor_wrapper(constructor) | indent}}
 }
 {% endmacro %}