Add a test for metrics collection + non-servable PrefetchContainer

After this CL, PrefetchServiceAlwaysBlockUntilHeadTest.
FailedTimeoutWhileBlockUntilHeadForOlderNavigation has
three navigations:

- First navigation is started but gone.
- Second navigation is started, blocked until head but
  the PrefetchContainer turned out not servable.
- Third navigation is started but the PrefetchContainer
  is not servable in the first place.

The metric
`PrefetchProxy.AfterClick.BlockUntilHeadDuration.NotServed`
should be recorded only for the second navigation.

The metric is currently recorded for the third navigation
but this will be fixed by
https://chromium-review.googlesource.com/c/chromium/src/+/4917584.

Bug: 1493192, 1448731
Change-Id: I4c9f4d7ddaeb67e3e2e783889349a949fd3a342d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4944370
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1211233}