layout: Make inner scroll wrappers acquire the overflow region of their

contents.

Makes Twitter pages show up when scrolling, if WebRender is not in use.
WebRender does not yet support overflow:scroll.

components/layout/display_list_builder.rs

@@ -1308,7 +1308,9 @@ impl FragmentDisplayListBuilding for Fragment {
             }
         };
         let overflow = match mode {
-            StackingContextCreationMode::InnerScrollWrapper |
+            StackingContextCreationMode::InnerScrollWrapper => {
+                Rect::new(Point2D::zero(), base_flow.overflow.paint.size)
+            }
             StackingContextCreationMode::OuterScrollWrapper => {
                 Rect::new(Point2D::zero(), border_box.size)
             }