PrefetchScheduler: Factor out `EvictPrefetch()`

This is a preparation of https://crbug.com/406402069.

This CL factors `PrefetchService::EvictPrefetch()` out in order to call
it from a new component.

Note that this CL changes the timing of eviction a little, but it
should make no behavioral changes.

Bug: 406403063, 406402069
Change-Id: I96d1fc3e0335e805519dadbf96aaae224f240b6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6402913
Commit-Queue: Ken Okada <kenoss@chromium.org>
Reviewed-by: Taiyo Mizuhashi <taiyo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1439185}