DataTransfer: Make |types| be a FrozenArray<DOMString>.

third_party/WebKit/LayoutTests/external/wpt/interfaces/html.idl

 // HTML IDLs
 typedef (Int8Array or Uint8Array or Uint8ClampedArray or
          Int16Array or Uint16Array or
          Int32Array or Uint32Array or
          Float32Array or Float64Array or
          DataView) ArrayBufferView;
 
 [NoInterfaceObject, Exposed=Window]
 interface HTMLHyperlinkElementUtils {
   stringifier attribute USVString href;
@@ skipping 1308 matching lines @@
 
 interface DataTransfer {
            attribute DOMString dropEffect;
            attribute DOMString effectAllowed;
 
   readonly attribute DataTransferItemList items;
 
   void setDragImage(Element image, long x, long y);
 
   /* old interface */
-  readonly attribute DOMString[] types;
+  readonly attribute FrozenArray<DOMString> types;
   DOMString getData(DOMString format);
   void setData(DOMString format, DOMString data);
   void clearData(optional DOMString format);
   readonly attribute FileList files;
 };
 
 interface DataTransferItemList {
   readonly attribute unsigned long length;
   getter DataTransferItem (unsigned long index);
   DataTransferItem? add(DOMString data, DOMString type);
@@ skipping 940 matching lines @@
   void captureEvents();
   void releaseEvents();
 
   readonly attribute HTMLAllCollection all;
 };
 
 partial interface Window {
   void captureEvents();
   void releaseEvents();
 };