Fix fixed position items with parents with CSS clips

In order to properly handle CSS clipping, we need to keep track of what
the different kinds of clips that we have. On one hand, clipping due to
overflow rules should respect the containing block hierarchy, while CSS
clipping should respect the flow tree hierarchy. In order to represent
the complexity of items that are scrolled via one clip/scroll frame and
clipped by another we keep track of that status with a
ClipAndScrollInfo.

components/msg/constellation_msg.rs

@@ -248,6 +248,10 @@ impl PipelineId {
     pub fn root_scroll_node(&self) -> webrender_api::ClipId {
         webrender_api::ClipId::root_scroll_node(self.to_webrender())
     }
+
+    pub fn root_clip_and_scroll_info(&self) -> webrender_api::ClipAndScrollInfo {
+        webrender_api::ClipAndScrollInfo::simple(self.root_scroll_node())
+    }
 }
 
 impl fmt::Display for PipelineId {