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/sslstreamadapter.h

@@ -228,9 +228,10 @@ class SSLStreamAdapter : public StreamAdapterInterface {
   // SS_OPENING but IsTlsConnected should return true.
   virtual bool IsTlsConnected() = 0;
 
-  // Capabilities testing.
-  // Used to have "DTLS supported", "DTLS-SRTP supported" etc. methods, but now
-  // that's assumed.
+  // Capabilities testing
+  static bool HaveDtls();
+  static bool HaveDtlsSrtp();
+  static bool HaveExporter();
   static bool IsBoringSsl();
 
   // Returns true iff the supplied cipher is deemed to be strong.