[Private Network Access] Test metric recording for cached resources.

This test currently shows that usecounters are incremented when a
document fetches a resource cached from a more-private IP address space.
This should not be the case, since these fetches are not affected by
Private Network Access checks yet.

The corresponding content browsertest was badly named, this CL fixes it.

Bug: chromium:1293764, chromium:1124340
Change-Id: I2c82f14ac75f7d244e4df45f5673a95161b0609f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3440002


Reviewed-by: Yifan Luo <lyf@chromium.org>
Auto-Submit: Titouan Rigoudy <titouan@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#968348}