[ServiceWorker+Prefetch] Plumb ServiceWorkerClient with RequestHandler

For the ServiceWorker-controlled prefetches, `ServiceWorkerClient` is
also a part of prefetched result (as the prefetch record's request's
reserved client).

This CL is plumbing only and doesn't change the behavior (especially
with the flag disabled, where the returned `ServiceWorkerClient` is
always null).

Bug: 40947546
Change-Id: Id96d5e2d0a08151e2d6f28a6f481091187ae156a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6250403
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Taiyo Mizuhashi <taiyo@chromium.org>
Reviewed-by: Ken Okada <kenoss@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432522}