RTCCertificate::Expires() and ::HasExpired() implemented

webrtc/base/sslidentity.h

Index: webrtc/base/sslidentity.h
diff --git a/webrtc/base/sslidentity.h b/webrtc/base/sslidentity.h
index b8063cee1569c0d7fea44f9a625e4f8e734cbbd3..a143ee4108bc4585174e2a19838133273751a5d9 100644
--- a/webrtc/base/sslidentity.h
+++ b/webrtc/base/sslidentity.h
@@ -70,7 +70,8 @@ class SSLCertificate {
                              size_t size,
                              size_t* length) const = 0;
 
-  // Returns the time in seconds relative to epoch.
+  // Returns the time in seconds relative to epoch, 1970-01-01T00:00:00Z (UTC),
+  // or -1 if an expiration time could not be retrieved.
   virtual int64_t CertificateExpirationTime() const = 0;
 };