Implement Windows GPO support for "dlpref".

This is a feature of the update protocol, which allows the
server to prioritize returning urls that can be cached
by downstream proxies.

BUG=579762

Review URL: https://codereview.chromium.org/1606943007

Cr-Commit-Position: refs/heads/master@{#371576}
20 files changed