Remove HistoryItem::m_frameSequenceNumber, it is unused

Source/core/loader/HistoryItem.h

 /*
  * Copyright (C) 2006, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Research In Motion Limited. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 78 matching lines @@
 
     void setStateObject(PassRefPtr<SerializedScriptValue>);
     SerializedScriptValue* stateObject() const { return m_stateObject.get(); }
 
     void setItemSequenceNumber(long long number) { m_itemSequenceNumber = number; }
     long long itemSequenceNumber() const { return m_itemSequenceNumber; }
 
     void setDocumentSequenceNumber(long long number) { m_documentSequenceNumber = number; }
     long long documentSequenceNumber() const { return m_documentSequenceNumber; }
 
-    void setFrameSequenceNumber(long long number) { m_frameSequenceNumber = number; }
-    long long frameSequenceNumber() const { return m_frameSequenceNumber; }
-
     void setScrollRestorationType(HistoryScrollRestorationType  type) { m_scrollRestorationType = type; }
     HistoryScrollRestorationType scrollRestorationType() { return m_scrollRestorationType; }
 
     void setFormInfoFromRequest(const ResourceRequest&);
     void setFormData(PassRefPtr<FormData>);
     void setFormContentType(const AtomicString&);
 
     bool isCurrentDocument(Document*) const;
 
     DECLARE_TRACE();
@@ skipping 15 matching lines @@
     // clones of one another. Traversing history from one such HistoryItem to
     // another is a no-op. HistoryItem clones are created for parent and
     // sibling frames when only a subframe navigates.
     int64_t m_itemSequenceNumber;
 
     // If two HistoryItems have the same document sequence number, then they
     // refer to the same instance of a document. Traversing history from one
     // such HistoryItem to another preserves the document.
     int64_t m_documentSequenceNumber;
 
-    // If two HistoryItems have the same frame sequence number, then they
-    // refer to the same instance of a Frame. This is used to determine whether
-    // whether a HistoryItem should navigate an existing frame or create a new
-    // one during a history navigation.
-    int64_t m_frameSequenceNumber;
-
     // Type of the scroll restoration for the history item determines if scroll
     // position should be restored when it is loaded during history traversal.
     HistoryScrollRestorationType m_scrollRestorationType;
 
     // Support for HTML5 History
     RefPtr<SerializedScriptValue> m_stateObject;
 
     // info used to repost form data
     RefPtr<FormData> m_formData;
     AtomicString m_formContentType;
 
 }; // class HistoryItem
 
 } // namespace blink
 
 #endif // HISTORYITEM_H