DsePreload2: Plumb NVS hint on WebContents::StartPrefetch()

This is a preparation of https://crrev.com/c/6239079.

DefaultSearchEngine prefetch uses two URLs: The first is to prefetch,
and the second is an user-initiated navigation or prerender. To handle
the difference, we need NVS hint/header. Currently, NVS hint is only
used for //content trigger. This CL admits to use NVS hint to embedder
triggers, including DefaultSearchEngine prefetch.

Bug: 394215571
Change-Id: Ice949b23db34bdc91b42936c0008911d1bda9abb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6238417
Commit-Queue: Ken Okada <kenoss@chromium.org>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1430123}