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

Source/core/html/HTMLButtonElement.h

Index: Source/core/html/HTMLButtonElement.h
diff --git a/Source/core/html/HTMLButtonElement.h b/Source/core/html/HTMLButtonElement.h
index dd12eb8a72c4b17929b47b5982fc2e4d480579e1..b88e28a33a504c977e77d9192fede7fe25af239f 100644
--- a/Source/core/html/HTMLButtonElement.h
+++ b/Source/core/html/HTMLButtonElement.h
@@ -30,7 +30,7 @@ namespace WebCore {
 
 class HTMLButtonElement : public HTMLFormControlElement {
 public:
-    static PassRefPtr<HTMLButtonElement> create(const QualifiedName&, Document*, HTMLFormElement*);
+    static Result<HTMLButtonElement> create(const QualifiedName&, Document*, HTMLFormElement*);
 
     void setType(const AtomicString&);
     
@@ -38,6 +38,8 @@ public:
 
     virtual bool willRespondToMouseClickEvents() OVERRIDE;
 
+    virtual void acceptHeapVisitor(Visitor*) const OVERRIDE;
+
 private:
     HTMLButtonElement(const QualifiedName& tagName, Document*, HTMLFormElement*);