<picture>: Update the <img> element when a <source> changes

Source/core/html/HTMLPictureElement.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef HTMLPictureElement_h
 #define HTMLPictureElement_h
 
 #include "core/html/HTMLElement.h"
 
 namespace WebCore {
 
 class HTMLPictureElement FINAL : public HTMLElement {
 public:
     DEFINE_NODE_FACTORY(HTMLPictureElement);
 
+    void sourceOrMediaChanged();
+
 protected:
     explicit HTMLPictureElement(Document&);
 };
 
 } // namespace
 
 #endif