Remove HistoryItem::m_frameSequenceNumber, it is unused

Source/core/loader/HistoryItem.h

Index: Source/core/loader/HistoryItem.h
diff --git a/Source/core/loader/HistoryItem.h b/Source/core/loader/HistoryItem.h
index 0124836de665cbf7ea7c0800070147b0f66df0ba..54e38dbf74a29be68f6f6ebd9ecb5cbde8b96861 100644
--- a/Source/core/loader/HistoryItem.h
+++ b/Source/core/loader/HistoryItem.h
@@ -96,9 +96,6 @@ public:
     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; }
 
@@ -134,12 +131,6 @@ private:
     // 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;