Fix several bugs related to scrolling

* scrollLeft/scrollTop returned values of parent or even document root
   Only the scroll of the node itself is returned. Otherwise 0.0.
* Scrolling via script had set viewport.
   This resulted in other nodes appearing scrolled.
   Now scroll_offsets are updated with correct node id.

These bugs caused other odd behavior like both body and
document.documentElement being scrolled or the view for scrolled
elements jumping.

components/script/dom/document.rs

@@ -703,6 +703,7 @@ impl Document {
             // Step 3
             let global_scope = self.window.upcast::<GlobalScope>();
             let webrender_pipeline_id = global_scope.pipeline_id().to_webrender();
+            self.window.update_viewport_for_scroll(x, y);
             self.window.perform_a_scroll(x,
                                          y,
                                          ClipId::root_scroll_node(webrender_pipeline_id),