Implement contextmenu attribute with basic support of <menu>

Source/core/html/HTMLElement.h

Index: Source/core/html/HTMLElement.h
diff --git a/Source/core/html/HTMLElement.h b/Source/core/html/HTMLElement.h
index 8a572c556109ead9e4075d7b093518159614dddd..6227ca91beabe6e0aead9ea18109379fd84cabb3 100644
--- a/Source/core/html/HTMLElement.h
+++ b/Source/core/html/HTMLElement.h
@@ -30,6 +30,7 @@ namespace blink {
 class DocumentFragment;
 class HTMLCollection;
 class HTMLFormElement;
+class HTMLMenuElement;
 class ExceptionState;
 
 enum TranslateAttributeMode {
@@ -93,6 +94,8 @@ public:
 
     static const AtomicString& eventParameterName();
 
+    HTMLMenuElement* menuElement() const;
+
 protected:
     HTMLElement(const QualifiedName& tagName, Document&, ConstructionType);