Delegate checking whether preconnect is enabled.

Currently preconnect can be disabled by the user through the
PrefService. As this is not a concept in //content, we need to extract
the code for checking to clients of PreconnectManager.

This is part of moving PreconnectManager to //content, see linked bug
for full plan.

Bug: 432235374
Change-Id: I03e5c051026561ad8288a02dd774b7b02c10e29c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6769821
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Keita Suzuki <suzukikeita@chromium.org>
Commit-Queue: Peter Conn <peconn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1490609}