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/base/nsPresShell.cpp

@@ -1309,9 +1309,6 @@ PresShell::Destroy()
     // to us. To avoid the pres context having a dangling reference, set its
     // pres shell to nullptr
     mPresContext->DetachShell();
-
-    // Clear the link handler (weak reference) as well
-    mPresContext->SetLinkHandler(nullptr);
   }
 
   mHaveShutDown = true;