Remove HistoryItem::m_frameSequenceNumber, it is unused

public/web/WebHistoryItem.h

 /*
  * Copyright (C) 2009 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 84 matching lines @@
 
     BLINK_EXPORT WebVector<WebString> documentState() const;
     BLINK_EXPORT void setDocumentState(const WebVector<WebString>&);
 
     BLINK_EXPORT long long itemSequenceNumber() const;
     BLINK_EXPORT void setItemSequenceNumber(long long);
 
     BLINK_EXPORT long long documentSequenceNumber() const;
     BLINK_EXPORT void setDocumentSequenceNumber(long long);
 
-    BLINK_EXPORT long long frameSequenceNumber() const;
-    BLINK_EXPORT void setFrameSequenceNumber(long long);
-
     BLINK_EXPORT WebHistoryScrollRestorationType scrollRestorationType() const;
     BLINK_EXPORT void setScrollRestorationType(WebHistoryScrollRestorationType);
 
     BLINK_EXPORT WebSerializedScriptValue stateObject() const;
     BLINK_EXPORT void setStateObject(const WebSerializedScriptValue&);
 
     BLINK_EXPORT WebString httpContentType() const;
     BLINK_EXPORT void setHTTPContentType(const WebString&);
 
     BLINK_EXPORT WebHTTPBody httpBody() const;
     BLINK_EXPORT void setHTTPBody(const WebHTTPBody&);
 
     BLINK_EXPORT WebVector<WebString> getReferencedFilePaths() const;
 
 #if BLINK_IMPLEMENTATION
     WebHistoryItem(const PassRefPtrWillBeRawPtr<HistoryItem>&);
     WebHistoryItem& operator=(const PassRefPtrWillBeRawPtr<HistoryItem>&);
     operator PassRefPtrWillBeRawPtr<HistoryItem>() const;
 #endif
 
 private:
     WebPrivatePtr<HistoryItem> m_private;
 };
 
 } // namespace blink
 
 #endif