Refactor PopularSites to use PrefService::GetValueList

This is part of a larger migration effort based on
go/trust-your-prefs.

Bug: 1341998
Test: Existing unit tests
Change-Id: I5399cc38a2c500faead216d4062da69c7e61db22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3865272


Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Roland Bock <rbock@google.com>
Cr-Commit-Position: refs/heads/main@{#1041703}