Use IDL argument default values in modules/encoding/

Source/modules/encoding/TextEncoder.idl

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 13 matching lines @@
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 [
     RuntimeEnabled=EncodingAPI,
     Exposed=Window&Worker,
-    Constructor(optional DOMString utfLabel = null),
+    Constructor(optional DOMString utfLabel = "utf-8"),
     RaisesException=Constructor,
     GarbageCollected,
     MeasureAs=TextEncoderConstructor
 ] interface TextEncoder {
     readonly attribute DOMString encoding;
-    [MeasureAs=TextEncoderEncode] Uint8Array encode(optional ScalarValueString input = null, optional Dictionary options);
+    [MeasureAs=TextEncoderEncode] Uint8Array encode(optional ScalarValueString input = "", optional Dictionary options);
 };