Preloading: Rename user agent override decision function

The `ShouldOverrideUserAgentForPrerender2()` function is used to
determine if the user agent should be overridden for preloading
navigations. Its name was specific to prerender, even though the
function is also used by other preloading mechanisms like prefetch.

This patch renames the function to more accurately reflect its general
use across all preloading types.

Bug: 427866914, 441612842
Change-Id: I8b7031b1604699f1e3f180fcdb2bfc17cce62209
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6993100
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Commit-Queue: Takashi Nakayama <tnak@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1522625}