Use DOMSettableTokenList for {HTMLAnchorElement, HTMLAreaElement}.ping.

Source/core/html/HTMLAnchorElement.h

Index: Source/core/html/HTMLAnchorElement.h
diff --git a/Source/core/html/HTMLAnchorElement.h b/Source/core/html/HTMLAnchorElement.h
index 142ac645069f5180f450d13b78c6d6096df1ad55..d975924389fdbaf3ca6947a4740841acc3c2694f 100644
--- a/Source/core/html/HTMLAnchorElement.h
+++ b/Source/core/html/HTMLAnchorElement.h
@@ -26,6 +26,7 @@
 
 #include "core/CoreExport.h"
 #include "core/HTMLNames.h"
+#include "core/dom/DOMSettableTokenList.h"
 #include "core/dom/DOMURLUtils.h"
 #include "core/dom/Document.h"
 #include "core/html/HTMLElement.h"
@@ -56,9 +57,10 @@ enum {
 //     RelationUp          = 0x00020000,
 };
 
-class CORE_EXPORT HTMLAnchorElement : public HTMLElement, public DOMURLUtils {
+class CORE_EXPORT HTMLAnchorElement : public HTMLElement, public DOMURLUtils, public DOMSettableTokenListObserver {
     DEFINE_WRAPPERTYPEINFO();
 public:
+    DECLARE_VIRTUAL_TRACE();
     static PassRefPtrWillBeRawPtr<HTMLAnchorElement> create(Document&);
 
     ~HTMLAnchorElement() override;
@@ -86,6 +88,8 @@ public:
 
     void sendPings(const KURL& destinationURL) const;
 
+    DOMSettableTokenList* ping() const;
+
 protected:
     HTMLAnchorElement(const QualifiedName&, Document&);
 
@@ -111,9 +115,12 @@ private:
     InsertionNotificationRequest insertedInto(ContainerNode*) override;
     void handleClick(Event*);
 
+    virtual void valueChanged() override;

[Mike West, 2015/07/13 13:55:07]

Nit: Add something like "// DOMSettableTokenListObserver" so we know what that's
implementing.

+
     uint32_t m_linkRelations;
     mutable LinkHash m_cachedVisitedLinkHash;
     bool m_wasFocusedByMouse;
+    RefPtrWillBeMember<DOMSettableTokenList> m_ping;
 };
 
 inline LinkHash HTMLAnchorElement::visitedLinkHash() const