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.