Fix formAction return value for empty formaction content attribute.

Source/core/html/HTMLFormControlElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 28 matching lines @@
 // HTMLFormControlElement is the default implementation of FormAssociatedElement,
 // and form-associated element implementations should use HTMLFormControlElement
 // unless there is a special reason.
 class CORE_EXPORT HTMLFormControlElement : public LabelableElement, public FormAssociatedElement {
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(HTMLFormControlElement);
 
 public:
     ~HTMLFormControlElement() override;
     DECLARE_VIRTUAL_TRACE();
 
+    String formAction() const;
+    void setFormAction(const AtomicString&);
     String formEnctype() const;
     void setFormEnctype(const AtomicString&);
     String formMethod() const;
     void setFormMethod(const AtomicString&);
     bool formNoValidate() const;
 
     void ancestorDisabledStateWasChanged();
 
     void reset();
 
@@ skipping 146 matching lines @@
 {
     return element.isFormControlElement();
 }
 
 DEFINE_HTMLELEMENT_TYPE_CASTS_WITH_FUNCTION(HTMLFormControlElement);
 DEFINE_TYPE_CASTS(HTMLFormControlElement, FormAssociatedElement, control, control->isFormControlElement(), control.isFormControlElement());
 
 } // namespace
 
 #endif