[oilpan] Completely move HTMLFormControlElement's hierarchy to the managed heap

Source/core/html/HTMLFormControlElement.h

Index: Source/core/html/HTMLFormControlElement.h
diff --git a/Source/core/html/HTMLFormControlElement.h b/Source/core/html/HTMLFormControlElement.h
index b9886d77bf5086b9ee960bf740202765a5b77e30..e2ff5e1c789ab4749194cd829308720c9fb0710f 100644
--- a/Source/core/html/HTMLFormControlElement.h
+++ b/Source/core/html/HTMLFormControlElement.h
@@ -30,7 +30,6 @@
 namespace WebCore {
 
 class FormDataList;
-class HTMLFieldSetElement;
 class HTMLFormElement;
 class HTMLLegendElement;
 class ValidationMessage;
@@ -99,13 +98,15 @@ public:
     bool hasAutofocused() { return m_hasAutofocused; }
     void setAutofocused() { m_hasAutofocused = true; }
 
-    static HTMLFormControlElement* enclosingFormControlElement(Node*);
+    static Result<HTMLFormControlElement> enclosingFormControlElement(Node*);
 
     using Node::ref;
     using Node::deref;
 
     virtual void reportMemoryUsage(MemoryObjectInfo*) const OVERRIDE;
 
+    virtual void acceptHeapVisitor(Visitor*) const OVERRIDE;
+
 protected:
     HTMLFormControlElement(const QualifiedName& tagName, Document*, HTMLFormElement*);