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/display_list/mod.rs

@@ -291,7 +291,7 @@ impl Fragment {
                             spatial_id: common.spatial_id,
                             clip_chain_id: common.clip_chain_id,
                         },
-                        iframe.pipeline_id.to_webrender(),
+                        iframe.pipeline_id.into(),
                         true,
                     );
                 },