chore: Clean up use of gfx and constellation types (#31981)

This change contains three semi-related clean ups:

1. the `to_webrender()` and `from_webrender()` functions on Pipeline are
   turned into more-idiomatic `From` and `Into` implementations.
2. `combine_id_with_fragment_type` now returns a `u64` as that is what is
   expected for all callers and not a `usize`.
3. The `query_scroll_id` query is removed entirely. The
   `ExternalScrollId` that this queries is easily generated directly
   from the node's opaque id. Querying into layout isn't necessary at
   all.

components/layout_2020/fragment_tree/base_fragment.rs

@@ -126,6 +126,6 @@ impl Tag {
             Some(PseudoElement::After) => FragmentType::AfterPseudoContent,
             _ => FragmentType::FragmentBody,
         };
-        combine_id_with_fragment_type(self.node.id(), fragment_type) as u64
+        combine_id_with_fragment_type(self.node.id(), fragment_type)
     }
 }