Expose CONNECT headers to ProxyDelegate

This brings back (slightly refreshed versions of) the
OnBeforeTunnelRequest(), OnTunnelHeadersReceived() hooks that were
removed in
https://chromium-review.googlesource.com/c/chromium/src/+/846261/.

These hooks are not used by the Data Reduction Proxy, because it
doesn't proxy https:// URLs.  They are restored for the benefit of those
downstream embedders (Opera) that use CONNECT tunnels to proxy HTTPS
requests.

Bug: 915659
Change-Id: Id0e120882a22a66c72802087a1775daeef661984
TBR: rockot@google.com
Reviewed-on: https://chromium-review.googlesource.com/c/1379766
Commit-Queue: Wojciech Dzier┼╝anowski <wdzierzanowski@opera.com>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624075}
48 files changed