Change "HTTPS proxy" => "supports QUIC" assumption to be opt-in

The Data Reduction Proxy servers are assumed to also support QUIC if
they are HTTPS proxies. This is currently not true for other custom
proxy config providers like Opera VPN.

Change-Id: I2c63a135ae21639eb98683a4476c9011c740feee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1511460
Commit-Queue: Wojciech Dzier┼╝anowski <wdzierzanowski@opera.com>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640780}
6 files changed