Provide method for returning certificate expiration timestamp.

webrtc/base/fakesslidentity.h

Index: webrtc/base/fakesslidentity.h
diff --git a/webrtc/base/fakesslidentity.h b/webrtc/base/fakesslidentity.h
index 7926580e7b4f03fcf185be5482930e0247faad07..69d590b5eb8fcd42483c5dd6cffa6821b4b99c3d 100644
--- a/webrtc/base/fakesslidentity.h
+++ b/webrtc/base/fakesslidentity.h
@@ -45,6 +45,7 @@ class FakeSSLCertificate : public rtc::SSLCertificate {
     VERIFY(SSLIdentity::PemToDer(kPemTypeCertificate, data_, &der_string));
     der_buffer->SetData(der_string.c_str(), der_string.size());
   }
+  int64_t CertificateExpirationTime() const override { return -1; }
   void set_digest_algorithm(const std::string& algorithm) {
     digest_algorithm_ = algorithm;
   }