Extensions: Add a metric to track a detailed reason for the proxy

Now Extensions.WebRequest.*Count can tell us how many Extensions require
each type of permissions per URLLoaderFactory, but could not know how
many cases of the proxy use can be reduced if each permission doesn't
require the proxy. This patch adds a metric that classifies a detailed
reason, i.e. only one type of permissions require it, or mixed reasons
exist.

Change-Id: I29afc3a2fea8ef0614bf19ce208dcbfb6d8f34cd
Bug: 391169957
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6277004
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1422351}