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 27946c5916c246c3fad01ed76b68b23c42353bb5..0773516c173607e1d9d9db5ab7b21d818a937894 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"
@@ -900,6 +901,24 @@ bool HTMLElement::isInteractiveContent() const
     return false;
 }
 
+HTMLMenuElement* HTMLElement::contextMenu() const
+{
+    const AtomicString& contextMenuId(fastGetAttribute(contextmenuAttr));
+    if (!contextMenuId.isNull()) {
+        Element* element = treeScope().getElementById(contextMenuId);
+        return isHTMLMenuElement(element) ? toHTMLMenuElement(element) : nullptr;
+    }
+
+    return nullptr;
+}
+
+void HTMLElement::setContextMenu(HTMLMenuElement* contextMenu)
+{
+    const AtomicString& contextMenuId(contextMenu->fastGetAttribute(idAttr));
+    if (!contextMenuId.isNull() && contextMenu->inDocument())
+        setAttribute(contextmenuAttr, contextMenuId);
+}
+
 void HTMLElement::defaultEventHandler(Event* event)
 {
     if (event->type() == EventTypeNames::keypress && event->isKeyboardEvent()) {