Oilpan: Prepare to move IdTargetObserver and IdTargetObserverRegistry to Oilpan heap.

Source/core/dom/TreeScope.cpp

Index: Source/core/dom/TreeScope.cpp
diff --git a/Source/core/dom/TreeScope.cpp b/Source/core/dom/TreeScope.cpp
index efe9c5045c73fb9c54aff8f6123f182da435fddc..709033ae3427d93db3d9d4db05fe624908ab5038 100644
--- a/Source/core/dom/TreeScope.cpp
+++ b/Source/core/dom/TreeScope.cpp
@@ -542,6 +542,7 @@ void TreeScope::trace(Visitor* visitor)
     visitor->trace(m_rootNode);
     visitor->trace(m_document);
     visitor->trace(m_parentTreeScope);
+    visitor->trace(m_idTargetObserverRegistry);
     visitor->trace(m_selection);
 }