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Modernize base::Value handling in a few ChromeOS configuration policy handlers

This is a follow-up to CL:1446226 .

Bug: none
Test: unit tests
Change-Id: Ia3e323a8b5ab9f18d69af944a9b85c479625b055
Reviewed-on: https://chromium-review.googlesource.com/c/1452187
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629974}