Add a frame sequence number to HistoryItem

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 80a31cb4dbfe128e925733de7646978127b613a2..9e1b3e2bd12205da327a7b29a50185692de0e5f1 100644
--- a/Source/core/loader/HistoryItem.h
+++ b/Source/core/loader/HistoryItem.h
@@ -94,6 +94,9 @@ 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 setFormInfoFromRequest(const ResourceRequest&);
     void setFormData(PassRefPtr<FormData>);
     void setFormContentType(const AtomicString&);
@@ -124,6 +127,12 @@ 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;
+
     // Support for HTML5 History
     RefPtr<SerializedScriptValue> m_stateObject;