layout: Allow non-absolutely-positioned elements with overflow: scroll

set to be scrolled.

This makes them establish stacking contexts, which is a CSS 2.1 spec
violation. However, we were already violating the spec here for
absolutely-positioned elements with `overflow: scroll`. It will probably
be easier to fix this spec violation once we either switch entirely to
WebRender or we have multiple layers per stacking context.

Closes #2742.

components/gfx_traits/lib.rs

@@ -105,6 +105,10 @@ impl LayerId {
         let LayerId(layer_type, id, _) = *self;
         LayerId(layer_type, id, 0)
     }
+
+    pub fn kind(&self) -> LayerType {
+        self.0
+    }
 }
 
 /// All layer-specific information that the painting task sends to the compositor other than the