Bug 1312742 - Match spec/other browsers for ping reflection; r=bkelly

The spec used to say that getting the .ping property on
HTMLAnchorElement or HTMLAreaElement should return a DOMTokenList of
parsed URLs, but it now says to return a plain string.  All other UAs
that implement ping match the new spec, and it's hopefully unlikely that
any sites depend on our old behavior.

Discussion: https://github.com/whatwg/html/issues/1780

MozReview-Commit-ID: LpmH8ANNT9g

dom/html/HTMLAnchorElement.cpp

@@ -347,10 +347,11 @@ HTMLAnchorElement::ToString(nsAString& aSource)
   return GetHref(aSource);
 }
 
-NS_IMETHODIMP    
+NS_IMETHODIMP
 HTMLAnchorElement::GetPing(nsAString& aValue)
 {
-  return GetURIListAttr(nsGkAtoms::ping, aValue);
+  GetAttr(kNameSpaceID_None, nsGkAtoms::ping, aValue);
+  return NS_OK;
 }
 
 NS_IMETHODIMP