Add a unique frame id and save it on HistoryItem.

Source/core/history/HistoryItem.h

Index: Source/core/history/HistoryItem.h
diff --git a/Source/core/history/HistoryItem.h b/Source/core/history/HistoryItem.h
index 15793db5257715c5c1aefb330499eba38d72e469..4bd3524f44a6daf47b4bbe4a952555f422ed4d5b 100644
--- a/Source/core/history/HistoryItem.h
+++ b/Source/core/history/HistoryItem.h
@@ -91,6 +91,9 @@ public:
     void setDocumentSequenceNumber(long long number) { m_documentSequenceNumber = number; }
     long long documentSequenceNumber() const { return m_documentSequenceNumber; }
 
+    void setTargetFrameID(int64_t id) { m_targetFrameID = id; }
+    int64_t targetFrameID() const { return m_targetFrameID; }
+
     void setFormInfoFromRequest(const ResourceRequest&);
     void setFormData(PassRefPtr<FormData>);
     void setFormContentType(const String&);
@@ -140,6 +143,8 @@ private:
     // such HistoryItem to another preserves the document.
     int64_t m_documentSequenceNumber;
 
+    int64_t m_targetFrameID;
+
     // Support for HTML5 History
     RefPtr<SerializedScriptValue> m_stateObject;