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).

dom/base/nsDocument.cpp

@@ -1288,6 +1288,7 @@ nsIDocument::nsIDocument()
 
 nsDocument::nsDocument(const char* aContentType)
   : nsIDocument()
+  , mLinksEnabled(true)
   , mViewportType(Unknown)
 {
   SetContentTypeInternal(nsDependentCString(aContentType));