[searchprefetch] Introduce trace trigger for unexpected SearchPrefetch failures

This CL adds a tracing trigger to investigate the failure of using a
SearchPrefetch loader. This tracing event will help to narrow down where
a loader is unexpectedly destroyed before serving to a user.

Bug: 413557424
Change-Id: I1d7278ba7e52f1b94b6b0c4e8e600b5255235a4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6518850
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Lingqi Chi <lingqi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457413}