Oilpan: keep FormData on the heap by default.

Source/core/xmlhttprequest/XMLHttpRequest.cpp

 /*
  *  Copyright (C) 2004, 2006, 2008 Apple Inc. All rights reserved.
  *  Copyright (C) 2005-2007 Alexey Proskuryakov <ap@webkit.org>
  *  Copyright (C) 2007, 2008 Julien Chaffraix <jchaffraix@webkit.org>
  *  Copyright (C) 2008, 2011 Google Inc. All rights reserved.
  *  Copyright (C) 2012 Intel Corporation
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
@@ skipping 830 matching lines @@
         send(data.getAsBlob(), exceptionState);
         return;
     }
 
     if (data.isDocument()) {
         send(data.getAsDocument().get(), exceptionState);
         return;
     }
 
     if (data.isFormData()) {
-        send(data.getAsFormData().get(), exceptionState);
+        send(data.getAsFormData(), exceptionState);
         return;
     }
 
     ASSERT(data.isString());
     send(data.getAsString(), exceptionState);
 }
 
 void XMLHttpRequest::send(ExceptionState& exceptionState)
 {
     InspectorInstrumentation::willSendXMLHttpRequest(executionContext(), url());
@@ skipping 1025 matching lines @@
     visitor->trace(m_responseDocumentParser);
     visitor->trace(m_progressEventThrottle);
     visitor->trace(m_upload);
     visitor->trace(m_blobLoader);
     XMLHttpRequestEventTarget::trace(visitor);
     DocumentParserClient::trace(visitor);
     ActiveDOMObject::trace(visitor);
 }
 
 } // namespace blink