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 9b3f37aa814447891776f8d9c3d72cbfe5c91324..990bc3b09d109cc0049efb18a141b2fc100ea42f 100644
--- a/Source/core/html/HTMLElement.h
+++ b/Source/core/html/HTMLElement.h
@@ -30,6 +30,7 @@ namespace WebCore {
 class DocumentFragment;
 class HTMLCollection;
 class HTMLFormElement;
+class HTMLMenuElement;
 class ExceptionState;
 
 enum TranslateAttributeMode {
@@ -91,6 +92,8 @@ public:
     virtual bool matchesReadOnlyPseudoClass() const OVERRIDE;
     virtual bool matchesReadWritePseudoClass() const OVERRIDE;
 
+    HTMLMenuElement* menuElement() const;
+
 protected:
     HTMLElement(const QualifiedName& tagName, Document&, ConstructionType);