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-Commit-Position: refs/heads/master@{#17369}
diff --git a/webrtc/base/proxy_unittest.cc b/webrtc/base/proxy_unittest.cc
new file mode 100644
index 0000000..b72d228
--- /dev/null
+++ b/webrtc/base/proxy_unittest.cc
@@ -0,0 +1,76 @@
+/*
+ *  Copyright 2009 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include <memory>
+#include <string>
+#include "webrtc/base/gunit.h"
+#include "webrtc/base/httpserver.h"
+#include "webrtc/base/proxyserver.h"
+#include "webrtc/base/socketadapters.h"
+#include "webrtc/base/testclient.h"
+#include "webrtc/base/testechoserver.h"
+#include "webrtc/base/virtualsocketserver.h"
+
+using rtc::Socket;
+using rtc::Thread;
+using rtc::SocketAddress;
+
+static const SocketAddress kSocksProxyIntAddr("1.2.3.4", 1080);
+static const SocketAddress kSocksProxyExtAddr("1.2.3.5", 0);
+static const SocketAddress kHttpsProxyIntAddr("1.2.3.4", 443);
+static const SocketAddress kHttpsProxyExtAddr("1.2.3.5", 0);
+static const SocketAddress kBogusProxyIntAddr("1.2.3.4", 999);
+
+// Sets up a virtual socket server and HTTPS/SOCKS5 proxy servers.
+class ProxyTest : public testing::Test {
+ public:
+  ProxyTest() : ss_(new rtc::VirtualSocketServer(nullptr)) {
+    Thread::Current()->set_socketserver(ss_.get());
+    socks_.reset(new rtc::SocksProxyServer(
+        ss_.get(), kSocksProxyIntAddr, ss_.get(), kSocksProxyExtAddr));
+    https_.reset(new rtc::HttpListenServer());
+    https_->Listen(kHttpsProxyIntAddr);
+  }
+  ~ProxyTest() { Thread::Current()->set_socketserver(nullptr); }
+
+  rtc::SocketServer* ss() { return ss_.get(); }
+
+ private:
+  std::unique_ptr<rtc::SocketServer> ss_;
+  std::unique_ptr<rtc::SocksProxyServer> socks_;
+  // TODO: Make this a real HTTPS proxy server.
+  std::unique_ptr<rtc::HttpListenServer> https_;
+};
+
+// Tests whether we can use a SOCKS5 proxy to connect to a server.
+TEST_F(ProxyTest, TestSocks5Connect) {
+  rtc::AsyncSocket* socket =
+      ss()->CreateAsyncSocket(kSocksProxyIntAddr.family(), SOCK_STREAM);
+  rtc::AsyncSocksProxySocket* proxy_socket =
+      new rtc::AsyncSocksProxySocket(socket, kSocksProxyIntAddr,
+                                           "", rtc::CryptString());
+  // TODO: IPv6-ize these tests when proxy supports IPv6.
+
+  rtc::TestEchoServer server(Thread::Current(),
+                                   SocketAddress(INADDR_ANY, 0));
+
+  rtc::AsyncTCPSocket* packet_socket = rtc::AsyncTCPSocket::Create(
+      proxy_socket, SocketAddress(INADDR_ANY, 0), server.address());
+  EXPECT_TRUE(packet_socket != nullptr);
+  rtc::TestClient client(packet_socket);
+
+  EXPECT_EQ(Socket::CS_CONNECTING, proxy_socket->GetState());
+  EXPECT_TRUE(client.CheckConnected());
+  EXPECT_EQ(Socket::CS_CONNECTED, proxy_socket->GetState());
+  EXPECT_EQ(server.address(), client.remote_address());
+  client.Send("foo", 3);
+  EXPECT_TRUE(client.CheckNextPacket("foo", 3, nullptr));
+  EXPECT_TRUE(client.CheckNoPacket());
+}