Move FontFaceSet.{check,load}'s argument default into IDL file

Source/core/css/FontFaceSet.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 22 matching lines @@
 [
     ActiveDOMObject,
     SetWrapperReferenceFrom=document,
     NoInterfaceObject,
 ] interface FontFaceSet : EventTarget {
 
     attribute EventHandler onloading;
     attribute EventHandler onloadingdone;
     attribute EventHandler onloadingerror;
 
-    [RaisesException] boolean check(DOMString font, optional DOMString text = null);
-    [CallWith=ScriptState] Promise load(DOMString font, optional DOMString text = null);
+    [RaisesException] boolean check(DOMString font, optional DOMString text = " ");
+    [CallWith=ScriptState] Promise load(DOMString font, optional DOMString text = " ");
     [MeasureAs=FontFaceSetReady, CallWith=ScriptState] Promise ready();
 
     [RaisesException] void add(FontFace fontFace);
     void clear();
     [RaisesException, ImplementedAs=remove] boolean delete(FontFace fontFace);
     void forEach(FontFaceSetForEachCallback callback, optional any thisArg);
     [RaisesException] boolean has(FontFace fontFace);
 
     readonly attribute unsigned long size;
     readonly attribute FontFaceSetLoadStatus status;
 };