Treat sessions as expired on the boundary second.

TLS 1.3 clarifies that a ticket lifetime of zero means the session is
unusable. We don't currently pay attention to that field (to be fixed in
later changes) but, in preparation for this, switch the >= to a >.

Change-Id: I0e67a0d97bc8def04914f121e84d3e7a2d640d2c
Reviewed-on: https://boringssl-review.googlesource.com/12102
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

ssl/ssl_session.c

@@ -605,7 +605,7 @@ int ssl_session_is_time_valid(const SSL *ssl, const SSL_SESSION *session) {
     return 0;
   }
 
-  return session->timeout >= (long)now.tv_sec - session->time;
+  return session->timeout > (long)now.tv_sec - session->time;
 }
 
 /* ssl_lookup_session looks up |session_id| in the session cache and sets

ssl/ssl_test.cc

@@ -2142,7 +2142,7 @@ static bool TestSessionTimeout() {
       }
 
       // Advance the clock just behind the timeout.
-      g_current_time.tv_sec += SSL_DEFAULT_SESSION_TIMEOUT;
+      g_current_time.tv_sec += SSL_DEFAULT_SESSION_TIMEOUT - 1;
 
       if (!ExpectSessionReused(client_ctx.get(), server_ctx.get(),
                                session.get(),