Prefetch: refactor: Replace callback with direct call OnGotEligibility

This CL refactors eligibility check of PrefetchService; stops using
callback, which is an obstruction of code jump, and uses direct call
of OnGotEligibility.

No behavioral change.

Bug: 343867153, 348338099
Unresolved-Comment-Reason: TODO is remained that is out of scope of this CL.
Change-Id: I6e7b5b11f216641392eb39da5cd42ea9c70164da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5644653
Auto-Submit: Ken Okada <kenoss@chromium.org>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Commit-Queue: Ken Okada <kenoss@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1332204}