Revert of Removing #defines previously used for building without BoringSSL/OpenSSL. (patchset #2 id:20001 of https://codereview.webrtc.org/2640513002/ )

Reason for revert:
Broke chromium build, due to a config being removed. Will add it back and remove the dependency in a chromium CL.

Original issue's description:
> Removing #defines previously used for building without BoringSSL/OpenSSL.
>
> These defines don't work any more, so they only cause confusion:
>
> FEATURE_ENABLE_SSL
> HAVE_OPENSSL_SSL_H
> SSL_USE_OPENSSL
>
> BUG=webrtc:7025
>
> Review-Url: https://codereview.webrtc.org/2640513002
> Cr-Commit-Position: refs/heads/master@{#16196}
> Committed: eaa826c2ee

TBR=kjellander@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7025

Review-Url: https://codereview.webrtc.org/2648003003
Cr-Commit-Position: refs/heads/master@{#16197}

webrtc/base/sslconfig.h

@@ -0,0 +1,30 @@
+/*
+ *  Copyright 2012 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.
+ */
+
+#ifndef WEBRTC_BASE_SSLCONFIG_H_
+#define WEBRTC_BASE_SSLCONFIG_H_
+
+// If no preference has been indicated, default to SChannel on Windows and
+// OpenSSL everywhere else, if it is available.
+#if !defined(SSL_USE_SCHANNEL) && !defined(SSL_USE_OPENSSL)
+#if defined(WEBRTC_WIN)
+
+#define SSL_USE_SCHANNEL 1
+
+#else  // defined(WEBRTC_WIN)
+
+#if defined(HAVE_OPENSSL_SSL_H)
+#define SSL_USE_OPENSSL 1
+#endif
+
+#endif  // !defined(WEBRTC_WIN)
+#endif
+
+#endif  // WEBRTC_BASE_SSLCONFIG_H_