OMTPrefetch: Add a test that PrePrefetchService handles non-ui headers callback

This CL adds a test that the `PrePrefetchUpdateHeadersCallback`s passed
to `PrePrefetchService::Create` are correctly executed on the non-UI
thread and their modifications to `ResourceRequest` headers are
correctly applied.

Bug: 452406598, 452389538
Change-Id: I6f5c030b06369e3dec53e85a420d9a2e7ee65b6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7864550
Commit-Queue: Taiyo Mizuhashi <taiyo@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1634797}