Introduce PrefetchResponseReader::LoadState

This CL makes `PrefetchResponseReader` state transitions
clearer, by introducing more explicit states
and replacing `last_event_added_` and `servable_`.

Bug: 1449360
Change-Id: Ib2a7ee0a8d9458c0a693e2c1292102e8ac2e2b7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4684835


Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Max Curran <curranmax@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1184813}