Unit test PolicyContainerPolicies::Clone()

This change adds a unit test for the PolicyContainerPolicies.

Bug: 1149272
Change-Id: I15d9a1395273e6ad4f26cbd285f0cc40eefa90a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2727505


Commit-Queue: Antonio Sartori <antoniosartori@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#859736}