Change the type of session_id() from string to int64_t.

webrtc/api/ortc/sessiondescription_unittest.cc

 /*
  *  Copyright 2017 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/api/ortc/sessiondescription.h"
 #include "webrtc/test/gtest.h"
 
 namespace webrtc {
 
 class SessionDescriptionTest : public testing::Test {};
 
 TEST_F(SessionDescriptionTest, CreateSessionDescription) {
-  SessionDescription s("a", "0");
-  EXPECT_EQ("a", s.session_id());
+  SessionDescription s(-1, "0");
+  EXPECT_EQ(-1, s.session_id());
   EXPECT_EQ("0", s.session_version());
 }
 }