Define "opaque" FormData objects.

Source/core/html/FormData.idl

 /*
  * Copyright (C) 2010 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 29 matching lines @@
     ImplementedAs=DOMFormData,
 ] interface FormData {
     // TODO(philipj): The value argument should be FormDataEntryValue and there
     // should be no optional filename argument. crbug.com/498790
     [CallWith=ExecutionContext] void append(USVString name, Blob value, optional USVString filename);
     void append(USVString name, USVString value);
 
     [RuntimeEnabled=FormDataNewMethods, ImplementedAs=deleteEntry] void delete(USVString name);
     [RuntimeEnabled=FormDataNewMethods] FormDataEntryValue? get(USVString name);
     [RuntimeEnabled=FormDataNewMethods] sequence<FormDataEntryValue> getAll(USVString name);
-    [RuntimeEnabled=FormDataNewMethods, ImplementedAs=hasEntry] boolean has(USVString name);
+    [RuntimeEnabled=FormDataNewMethods] boolean has(USVString name);
     // TODO(philipj): The value argument should be FormDataEntryValue and there
     // should be no optional filename argument.
     [RuntimeEnabled=FormDataNewMethods] void set(USVString name, Blob value, optional USVString filename);
     [RuntimeEnabled=FormDataNewMethods] void set(USVString name, USVString value);
     [RuntimeEnabled=FormDataNewMethods] iterable<USVString, FormDataEntryValue>;
 };