Issue #2721 - Create special case exception for <A>.Click() outside of DOM

This removes the requirement for there to be a non-null PresShell to
dispatch `Click()` events on `<A>` elements (only), since the exception
to the rule has propagated to the spec.

With these changes it should now be possible do create an anchor and
`Click()` on it from JS without actually first attaching it to the DOM
of the presented document, as abused by scripted downloads in pages
(instead of using the A attribute to custom-name downloads).

layout/generic/nsImageFrame.cpp

@@ -2046,7 +2046,7 @@ nsImageFrame::HandleEvent(nsPresContext* aPresContext,
             *aEventStatus = nsEventStatus_eConsumeDoDefault;
             clicked = true;
           }
-          nsContentUtils::TriggerLink(anchorNode, aPresContext, uri, target,
+          nsContentUtils::TriggerLink(anchorNode, uri, target,
                                       clicked, true, true);
         }
       }