[Fixit] Use mock policy provider utility in remaining dirs (3/3).

A utility method was implemented which reduces the amount of boilerplate
code required when using MockConfigurationPolicyProvider.

NiceMock<...> is used to suppress the "Uninteresting mock function call"
warnings.

Utility method implementation CL: crrev.com/c/3017141

This cleanup is part of Chrome Fixit 2021.

Bug: 242087
Change-Id: I4aa86fe17a223aa9fc8d9965307ceefbc7e4d1b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3032642
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Commit-Queue: Igor Ruvinov <igorruvinov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#903470}