Should clear sheet when ProcessingInstruction's removedFrom or attributeChanged

Source/core/dom/ProcessingInstruction.h

 /*
  * Copyright (C) 2000 Peter Kelly (pmk@post.com)
  * Copyright (C) 2006 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 53 matching lines @@
 
     bool checkStyleSheet(String& href, String& charset);
     void process(const String& href, const String& charset);
 
     virtual void setCSSStyleSheet(const String& href, const KURL& baseURL, const String& charset, const CSSStyleSheetResource*) OVERRIDE;
     virtual void setXSLStyleSheet(const String& href, const KURL& baseURL, const String& sheet) OVERRIDE;
 
     virtual bool sheetLoaded() OVERRIDE;
 
     void parseStyleSheet(const String& sheet);
+    void clearSheet();
 
     String m_target;
     String m_localHref;
     String m_title;
     String m_media;
     RefPtrWillBeMember<StyleSheet> m_sheet;
     bool m_loading;
     bool m_alternate;
     bool m_createdByParser;
     bool m_isCSS;
     bool m_isXSL;
 };
 
 DEFINE_NODE_TYPE_CASTS(ProcessingInstruction, nodeType() == Node::PROCESSING_INSTRUCTION_NODE);
 
 inline bool isXSLStyleSheet(const Node& node)
 {
     return node.nodeType() == Node::PROCESSING_INSTRUCTION_NODE && toProcessingInstruction(node).isXSL();
 }
 
 } // namespace blink
 
 #endif