Use custom user data to serialize Tab last visited timestamp.

ios/web/navigation/crw_session_storage_unittest.mm

Index: ios/web/navigation/crw_session_storage_unittest.mm
diff --git a/ios/web/navigation/crw_session_storage_unittest.mm b/ios/web/navigation/crw_session_storage_unittest.mm
index 4bbd394d70f46aacaa5a01346eab28fb56e843ea..bcf3fd4c2aad077f5756f8bf2d797bb7f65636b0 100644
--- a/ios/web/navigation/crw_session_storage_unittest.mm
+++ b/ios/web/navigation/crw_session_storage_unittest.mm
@@ -51,7 +51,6 @@ BOOL SessionStoragesAreEqual(CRWSessionStorage* session1,
          session1.openedByDOM == session2.openedByDOM &&
          session1.currentNavigationIndex == session2.currentNavigationIndex &&
          session1.previousNavigationIndex == session2.previousNavigationIndex &&
-         session1.lastVisitedTimestamp == session2.lastVisitedTimestamp &&
          UserDataAreEqual(session1.userData, session2.userData);
 }
 }  // namespace
@@ -64,7 +63,6 @@ class CRWNSessionStorageTest : public PlatformTest {
     [session_storage_ setOpenedByDOM:YES];
     [session_storage_ setCurrentNavigationIndex:4];
     [session_storage_ setPreviousNavigationIndex:3];
-    [session_storage_ setLastVisitedTimestamp:CFAbsoluteTimeGetCurrent()];
     // Create an item storage.
     base::scoped_nsobject<CRWNavigationItemStorage> item_storage(
         [[CRWNavigationItemStorage alloc] init]);