Oilpan: keep FormData on the heap by default.

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 17 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 // http://xhr.spec.whatwg.org/#interface-formdata
 
 typedef (File or USVString) FormDataEntryValue;
 
 [
     Constructor(optional HTMLFormElement form),
     Exposed=(Window,Worker),
+    GarbageCollected,
     ImplementedAs=DOMFormData,
-    WillBeGarbageCollected,
 ] interface FormData {
     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] void set(USVString name, Blob value, optional USVString filename);
     [RuntimeEnabled=FormDataNewMethods] void set(USVString name, USVString value);
     [RuntimeEnabled=FormDataNewMethods] iterable<USVString, FormDataEntryValue>;
 };