Implement contextmenu attribute with basic support of <menu>

Source/core/html/HTMLElement.cpp

Index: Source/core/html/HTMLElement.cpp
diff --git a/Source/core/html/HTMLElement.cpp b/Source/core/html/HTMLElement.cpp
index 1a98cf035b19329dd366f97188a7a7072d8790bd..dd1059ed9fd093adaf18eb368d298a3e72da1a23 100644
--- a/Source/core/html/HTMLElement.cpp
+++ b/Source/core/html/HTMLElement.cpp
@@ -46,6 +46,7 @@
 #include "core/html/HTMLBRElement.h"
 #include "core/html/HTMLFormElement.h"
 #include "core/html/HTMLInputElement.h"
+#include "core/html/HTMLMenuElement.h"
 #include "core/html/HTMLTemplateElement.h"
 #include "core/html/HTMLTextFormControlElement.h"
 #include "core/html/parser/HTMLParserIdioms.h"
@@ -917,6 +918,17 @@ bool HTMLElement::isInteractiveContent() const
     return false;
 }
 
+HTMLMenuElement* HTMLElement::menuElement() const
+{
+    Element* element = treeScope().getElementById(fastGetAttribute(contextmenuAttr));
+    if (!element)
+        return 0;
+    if (!isHTMLMenuElement(*element))
+        return 0;
+
+    return toHTMLMenuElement(element);
+}
+
 void HTMLElement::defaultEventHandler(Event* event)
 {
     if (event->type() == EventTypeNames::keypress && event->isKeyboardEvent()) {