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 590b8eff543b5b1692d5a03681e742c48e930649..49f647be9d241b9810d523aa826f4758d183927b 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 {
@@ -95,6 +96,9 @@ public:
 
     static const AtomicString& eventParameterName();
 
+    HTMLMenuElement* contextMenu() const;
+    void setContextMenu(HTMLMenuElement*);
+
 protected:
     HTMLElement(const QualifiedName& tagName, Document&, ConstructionType);