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

webrtc/api/ortc/sessiondescription_unittest.cc

Index: webrtc/api/ortc/sessiondescription_unittest.cc
diff --git a/webrtc/api/ortc/sessiondescription_unittest.cc b/webrtc/api/ortc/sessiondescription_unittest.cc
index c856ea944e978d726f81926176fed1035efdcdaa..00d91980622147a9c6ee6e44278134b29af69a21 100644
--- a/webrtc/api/ortc/sessiondescription_unittest.cc
+++ b/webrtc/api/ortc/sessiondescription_unittest.cc
@@ -16,8 +16,8 @@ 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());
 }
 }