Remove HistoryItem::m_frameSequenceNumber, it is unused

Source/core/loader/HistoryItem.cpp

 /*
  * Copyright (C) 2005, 2006, 2008, 2011 Apple 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:
  * 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 30 matching lines @@
     // Initialize to the current time to reduce the likelihood of generating
     // identifiers that overlap with those from past/future browser sessions.
     static long long next = static_cast<long long>(currentTime() * 1000000.0);
     return ++next;
 }
 
 HistoryItem::HistoryItem()
     : m_pageScaleFactor(0)
     , m_itemSequenceNumber(generateSequenceNumber())
     , m_documentSequenceNumber(generateSequenceNumber())
-    , m_frameSequenceNumber(generateSequenceNumber())
     , m_scrollRestorationType(ScrollRestorationAuto)
 {
 }
 
 HistoryItem::~HistoryItem()
 {
 }
 
 void HistoryItem::generateNewItemSequenceNumber()
 {
@@ skipping 155 matching lines @@
     // FIXME: We should find a better way to check if this is the current document.
     return equalIgnoringFragmentIdentifier(url(), doc->url());
 }
 
 DEFINE_TRACE(HistoryItem)
 {
     visitor->trace(m_documentState);
 }
 
 } // namespace blink