Refactor WindowSizeAutosaver to use PrefService::GetValueDict

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

Bug: 1341996
Test: Existing unit tests
Change-Id: Iaedac96ae6dbec700c8a087d90a4b41617249f4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3864675


Commit-Queue: Roland Bock <rbock@google.com>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1042057}