Implement OpenSSLCertificate::GetChain

Continue from https://codereview.webrtc.org/3010363002/

BUG=webrtc:8289

Change-Id: I8d14ba7974b654387f63ff30dee822b3045edcc7
Reviewed-on: https://webrtc-review.googlesource.com/6500
Commit-Queue: Jian Cui <jiancui@google.com>
Reviewed-by: David Benjamin <davidben@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20723}
diff --git a/rtc_base/sslidentity.cc b/rtc_base/sslidentity.cc
index 79e2ad5..e035d9e 100644
--- a/rtc_base/sslidentity.cc
+++ b/rtc_base/sslidentity.cc
@@ -265,6 +265,13 @@
   return OpenSSLIdentity::FromPEMStrings(private_key, certificate);
 }
 
+// static
+SSLIdentity* SSLIdentity::FromPEMChainStrings(
+    const std::string& private_key,
+    const std::string& certificate_chain) {
+  return OpenSSLIdentity::FromPEMChainStrings(private_key, certificate_chain);
+}
+
 bool operator==(const SSLIdentity& a, const SSLIdentity& b) {
   return static_cast<const OpenSSLIdentity&>(a) ==
          static_cast<const OpenSSLIdentity&>(b);