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 d5c9c754a92cbefc8fa757afb46179696f09b339..784e34af3dc3fcf45919f0c28f955851b8b5c86e 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 +901,17 @@ bool HTMLElement::isInteractiveContent() const
     return false;
 }
 
+PassRefPtrWillBeRawPtr<HTMLMenuElement> HTMLElement::contextMenu() const
+{
+    Element* element = treeScope().getElementById(fastGetAttribute(contextmenuAttr));
+    return isHTMLMenuElement(element) ? toHTMLMenuElement(element) : 0;
+}
+
+void HTMLElement::setContextMenu(PassRefPtrWillBeRawPtr<HTMLMenuElement> contextMenu)
+{
+    setAttribute(contextmenuAttr, contextMenu->elementData()->idForStyleResolution());

[tkent, 2014/07/31 06:07:55]

Does this behavior conform to the standard?

[pals, 2014/08/01 10:03:07]

Specification says that the contextmenu attribute value must be the ID of a menu
element in the same home subtree. So, added the treescope check.
Do you mean the same? 

[tkent, 2014/08/04 02:26:12]

The HTML specification must have a detailed algorithm to reflect HTMLElement
object to ID content attribute.  Does your code conform to the algorithm?

I don't think your code is correct.  e.g. HTMLElement without ID, multiple
HTMLElement with simple ID.

[pals, 2014/08/04 08:58:27]

Now I am checking if id is null and  I have added tests with HTMLMenuElement
without ID, multiple HTMLMenuElement with same ID.
I am not sure if it still conforms to spec. Can you please point me to the
specification link which you are referring to? or some existing blink code which
implements similar spec? or some test code which should fails with the current
code?

+}
+
 void HTMLElement::defaultEventHandler(Event* event)
 {
     if (event->type() == EventTypeNames::keypress && event->isKeyboardEvent()) {