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: https://chromium.googlesource.com/external/webrtc/+/eaa826c2ee0668cfb4a0dfb66f8d388b65da20f5

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}
diff --git a/webrtc/base/sslidentity.cc b/webrtc/base/sslidentity.cc
index a5dd7b9..645050a 100644
--- a/webrtc/base/sslidentity.cc
+++ b/webrtc/base/sslidentity.cc
@@ -17,9 +17,15 @@
 #include "webrtc/base/base64.h"
 #include "webrtc/base/checks.h"
 #include "webrtc/base/logging.h"
-#include "webrtc/base/opensslidentity.h"
+#include "webrtc/base/sslconfig.h"
 #include "webrtc/base/sslfingerprint.h"
 
+#if SSL_USE_OPENSSL
+
+#include "webrtc/base/opensslidentity.h"
+
+#endif  // SSL_USE_OPENSSL
+
 namespace rtc {
 
 const char kPemTypeCertificate[] = "CERTIFICATE";
@@ -207,6 +213,8 @@
   std::for_each(certs_.begin(), certs_.end(), DeleteCert);
 }
 
+#if SSL_USE_OPENSSL
+
 // static
 SSLCertificate* SSLCertificate::FromPEMString(const std::string& pem_string) {
   return OpenSSLCertificate::FromPEMString(pem_string);
@@ -252,6 +260,12 @@
   return !(a == b);
 }
 
+#else  // !SSL_USE_OPENSSL
+
+#error "No SSL implementation"
+
+#endif  // SSL_USE_OPENSSL
+
 // Read |n| bytes from ASN1 number string at *|pp| and return the numeric value.
 // Update *|pp| and *|np| to reflect number of read bytes.
 static inline int ASN1ReadInt(const unsigned char** pp, size_t* np, size_t n) {