Make more [TreatNullAs=NullString] DOMString arguments nullable

Source/core/html/HTMLCanvasElement.idl

 /*
  * Copyright (C) 2006, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 11 matching lines @@
  * 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.
  */
 
 interface HTMLCanvasElement : HTMLElement {
 
     attribute long width;
     attribute long height;
 
-    [Custom, RaisesException] DOMString toDataURL([TreatNullAs=NullString, TreatUndefinedAs=NullString,Default=Undefined] optional DOMString type);
+    [Custom, RaisesException] DOMString toDataURL([TreatUndefinedAs=NullString, Default=Undefined] optional DOMString? type);
 
     // The custom binding is needed to handle context creation attributes.
     [Custom] any getContext([Default=Undefined] optional DOMString contextId);
 };