Prefetch: Derive net::RequestPriority from PrefetchPriority

If the prefetch caller expects to need prefetch resources to be
available sooner, we can give higher `net::RequestPriority` so that
their network request can be prioritized in //net layer.

Please see
https://docs.google.com/document/d/1Ifwra6N4URhs4yD3FS_AOuzB12rZGhfuphvRd2_wuzE/edit?tab=t.0#bookmark=id.742ztsc9wg3h
for more detailed mappings discussion between `PrefetchPriority` and
`net::RequestPriority` for the future. Currently `net::RequestPriority`
is calculated from `PrefetchType`, but it can eventually derived from
`PrefetchPriority`.

There is no behavior change in this CL, since
`PrefetchContainer::priority_` is always `std::nullopt` for now.

Bug: 426404355, 406403063
Change-Id: I528ea0566bcc6dd03309bece1cfb3a2905780807
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6654214
Reviewed-by: Ken Okada <kenoss@chromium.org>
Commit-Queue: Taiyo Mizuhashi <taiyo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1477173}