Modernize {Dictionary,List}PrefUpdate in /chrome/browser/chromeos/policy/dlp

This cl replaces instances of DictionaryPrefUpdateDeprecated, which
wraps the deprecated base::DictionaryValue, with DictionaryPrefUpdate,
which wraps base::Value. This cl also does the equivalent for
ListPrefUpdateDeprecated. A range of other base::Value modernization
changes are made to allow these replacements and as drive-by fixes.
This cl handles the directory /chrome/browser/chromeos/policy/dlp

This CL was uploaded by git cl split.

R=ayaelattar@chromium.org

Bug: 1285745
Change-Id: I272ff3e7152d9aef12fc1affe46fcf87ae7b9dc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3385996
Auto-Submit: Alex Turner <alexmt@chromium.org>
Reviewed-by: Aya Elsayed <ayaelattar@chromium.org>
Commit-Queue: Aya Elsayed <ayaelattar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#958511}