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 a185b749563cc387d0534fb16b52cfcada42e549..052b59879b23ae0c162e98e560b410881039ea2e 100644
--- a/Source/core/html/HTMLElement.cpp
+++ b/Source/core/html/HTMLElement.cpp
@@ -48,6 +48,7 @@
 #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"
@@ -892,6 +893,12 @@ bool HTMLElement::isInteractiveContent() const
     return false;
 }
 
+HTMLMenuElement* HTMLElement::menuElement() const
+{
+    Element* element = treeScope().getElementById(fastGetAttribute(contextmenuAttr));
+    return isHTMLMenuElement(element) ? toHTMLMenuElement(element) : 0;
+}
+
 void HTMLElement::defaultEventHandler(Event* event)
 {
     if (event->type() == EventTypeNames::keypress && event->isKeyboardEvent()) {