SearchPre*: SearchPrefetch triggered prerenders no longer need the
prefetch parameter.

SearchPrerender requests' URLs may contain a prefetch parameter respecting the search service URL templates. This causes some extra
work.

While chrome is running behind the `SearchPrefetchUpgradeToPrerender`
flag, SearchPrerender never sends main resource requests to search
service, so it doesn't need to obey the rules defined in the templates
and can remove this parameter.

This CL helps SearchPrerender get rid of the prefetch parameter and
adds some tests accordingly.

Utility updates:
This CL also updates the logic the handle search requests, allowing
test fixtures to determine when to respond response header and body by
introducing DelayedResponse and delayed response queue.

This is at the third step:
1. unify triggering path
2. trigger prerender after prefetch responses succeeds
3. * make prerender reuse the prefetched response
3.1 manage the state machine: for now we remove the prerendered
    prefetch requests upon users click the ACMatch, which varies from
    prefetch.
3.2 ensure timestamps are recorded correctly.
3.3 add timer to cancel prerender on expiration.
3.4 fix some edge cases and ensure prerender requests cannot enter net
    stack.
3.5 * remove the prefetch parameter from prerender's url, as prerender
    requests no longer enter net stack.
4. prefetch cancels prerender when needed.
5. metrics
6. allow prerender to fallback to prefetch when prerender is canceled.
6.1 manage the response data reader.

Bug: 1295170
Validate-Test-Flakiness: skip
Change-Id: Id39f6ed1bea414afbbfc5ac2df9e158358bdd59b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3715477
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Lingqi Chi <lingqi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1021699}