Add unittests checking that old saved session can be restored.

ios/chrome/browser/tabs/tab_model_unittest.mm

Index: ios/chrome/browser/tabs/tab_model_unittest.mm
diff --git a/ios/chrome/browser/tabs/tab_model_unittest.mm b/ios/chrome/browser/tabs/tab_model_unittest.mm
index 2fe10d7221417490241fe76ac0ec2f8a367d40bf..0daca89108d458404bd474ec3f3e6870d66f1f4b 100644
--- a/ios/chrome/browser/tabs/tab_model_unittest.mm
+++ b/ios/chrome/browser/tabs/tab_model_unittest.mm
@@ -41,17 +41,6 @@
 
 using web::WebStateImpl;
 
-// For some of the unit tests, we need to make sure the session is saved
-// immediately so we can read it back in to verify various attributes. This
-// is not a situation we normally expect to be in because we never
-// want the session being saved on the main thread in the production app.
-// We could expose this as part of the service's public API, but again that
-// might encourage use where we don't want it. As a result, just use the
-// known private-for-testing method directly.
-@interface SessionServiceIOS (Testing)
-- (void)performSaveWindow:(SessionWindowIOS*)window
-              toDirectory:(NSString*)directory;
-@end
 
 @interface TabModel (VisibleForTesting)
 - (SessionWindowIOS*)windowForSavingSession;