[oilpan] The Node hierarchy should have correct accept method chains

Source/core/html/LabelableElement.cpp

Index: Source/core/html/LabelableElement.cpp
diff --git a/Source/core/html/LabelableElement.cpp b/Source/core/html/LabelableElement.cpp
index aaeafd529e2aa732464054aa08ef456f0e8d4fee..2077d2cb641a7b029d4173936eb1361ae10fe267 100644
--- a/Source/core/html/LabelableElement.cpp
+++ b/Source/core/html/LabelableElement.cpp
@@ -50,4 +50,9 @@ PassRefPtr<NodeList> LabelableElement::labels()
     return ensureRareData()->ensureNodeLists()->addCacheWithAtomicName<LabelsNodeList>(this, LabelsNodeListType, starAtom);
 }
 
+void LabelableElement::acceptHeapVisitor(Visitor* visitor) const
+{
+    HTMLElement::acceptHeapVisitor(visitor);
+}
+
 } // namespace Webcore