IDL: Support optional argument default value syntax

Source/modules/encoding/TextEncoder.idl

Index: Source/modules/encoding/TextEncoder.idl
diff --git a/Source/modules/encoding/TextEncoder.idl b/Source/modules/encoding/TextEncoder.idl
index 243d2f8a03d26e14578f8fa626f86ed444950f74..e621bc97f831094a94a46c40792b859fc4e10c87 100644
--- a/Source/modules/encoding/TextEncoder.idl
+++ b/Source/modules/encoding/TextEncoder.idl
@@ -31,11 +31,11 @@
 [
     RuntimeEnabled=EncodingAPI,
     Exposed=Window&Worker,
-    Constructor([Default=NullString] optional DOMString utfLabel),
+    Constructor(optional DOMString utfLabel = null),
     RaisesException=Constructor,
     GarbageCollected,
     MeasureAs=TextEncoderConstructor
 ] interface TextEncoder {
     readonly attribute DOMString encoding;
-    [MeasureAs=TextEncoderEncode] Uint8Array encode([Default=NullString] optional DOMString input, optional Dictionary options);
+    [MeasureAs=TextEncoderEncode] Uint8Array encode(optional DOMString input = null, optional Dictionary options);
 };