Revert of Removing HTTPS and SOCKS proxy server code. (patchset #2 id:20001 of https://codereview.webrtc.org/2731673002/ )

Also needed to revert these CLs, which removed code used by the
code being un-removed:
https://codereview.webrtc.org/2745523004
https://codereview.webrtc.org/2754033003
https://codereview.webrtc.org/2758943002

Reason for revert:
This code is still being used by native application developers, so we should send a PSA announcing the deprecation and suggest an alternative before removing it.

Original issue's description:
> Removing HTTPS and SOCKS proxy server code.
>
> This isn't used any more so there's no point in maintaining it.
>
> BUG=None
>
> Review-Url: https://codereview.webrtc.org/2731673002
> Cr-Commit-Position: refs/heads/master@{#17016}
> Committed: https://chromium.googlesource.com/external/webrtc/+/a1991c517598fda9c9c0cf8876886e3968436ff9

TBR=pthatcher@webrtc.org
NOPRESUBMIT=true
NOTRY=true
BUG=None

Review-Url: https://codereview.webrtc.org/2766063005
Cr-Original-Commit-Position: refs/heads/master@{#17369}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f137e97adbc00895c6485fb8ddee407312d0d775
diff --git a/base/proxyserver.cc b/base/proxyserver.cc
index fd36ef1..713a986 100644
--- a/base/proxyserver.cc
+++ b/base/proxyserver.cc
@@ -149,4 +149,8 @@
   SignalDestroyed(this);
 }
 
+AsyncProxyServerSocket* SocksProxyServer::WrapSocket(AsyncSocket* socket) {
+  return new AsyncSocksProxyServerSocket(socket);
+}
+
 }  // namespace rtc