Store JavaScript bytecode side data in the WebContent HTTP memory
cache and replay it when serving cached responses. Also update an
already-complete memory-cache entry when asynchronous bytecode cache
generation finishes, so the first source-only response does not keep
shadowing the disk-cache sidecar during same-process navigations.
Keep the HTTP memory-cache backfill keyed with the request headers that
populated the memory-cache entry, so Vary responses still receive their
generated bytecode sidecar.
Add LibHTTP coverage for round-tripping bytecode side data through a
memory-cache entry, attaching it after the response body has already
been cached, and matching Vary headers during updates. Add LibWeb
coverage for preserving the memory-cache request headers when cloning
responses.
