Split the frame tree logic out of HistoryItem

Source/web/WebHistoryItem.cpp

Index: Source/web/WebHistoryItem.cpp
diff --git a/Source/web/WebHistoryItem.cpp b/Source/web/WebHistoryItem.cpp
index 4bb9746f84544f5b9ea026016ece239186c42c2a..807b20f541f70dca9e0629ef32defaf05f760fa2 100644
--- a/Source/web/WebHistoryItem.cpp
+++ b/Source/web/WebHistoryItem.cpp
@@ -237,6 +237,14 @@ WebVector<WebHistoryItem> WebHistoryItem::children() const
     return m_private->children();
 }
 
+void WebHistoryItem::setChildren(const WebVector<WebHistoryItem>& items)
+{
+    ensureMutable();
+    m_private->clearChildren();
+    for (size_t i = 0; i < items.size(); ++i)
+        m_private->addChildItem(items[i]);
+}
+
 void WebHistoryItem::appendToChildren(const WebHistoryItem& item)
 {
     ensureMutable();