Add code to allow HttpProxyClientSocketWrapper to wrap around a QuicProxyClientSocket.

This is working towards supporting HTTPS tunneling over QUIC in Chrome.

Bug: 335275
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I194538f5046d1d6fb0ca975d3c678270d8e6021a
Reviewed-on: https://chromium-review.googlesource.com/759758
Commit-Queue: Yixin Wang <wangyix@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515701}
12 files changed