Allow CustomProxyConfig to override existing config

A CustomProxyConfig provider like Opera VPN will now be able to specify
that the custom config must take priority over all other sources of
proxy configuration.  The default behavior is still the one required by
the Data Reduction Proxy: the DRP config is only effective if nothing
else provided a proxy config.

Change-Id: I4cc0a61375b668ae1806bb6978b3e0893ed941ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1491252
Commit-Queue: Wojciech Dzier┼╝anowski <wdzierzanowski@opera.com>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640707}
4 files changed