Add unittests checking that old saved session can be restored.

ios/chrome/browser/sessions/session_service.h

Index: ios/chrome/browser/sessions/session_service.h
diff --git a/ios/chrome/browser/sessions/session_service.h b/ios/chrome/browser/sessions/session_service.h
index 661b6779b83ab92f31db282acb6daae9b8673a2f..0676b2e4298b6e729a7d8911c95cc79bb859ec67 100644
--- a/ios/chrome/browser/sessions/session_service.h
+++ b/ios/chrome/browser/sessions/session_service.h
@@ -25,7 +25,8 @@ class ChromeBrowserState;
 // Inits exactly as initForReadingWithData: does, additionally setting
 // |browserState| on the receiver.
 - (instancetype)initForReadingWithData:(NSData*)data
-                          browserState:(ios::ChromeBrowserState*)browserState;
+                          browserState:(ios::ChromeBrowserState*)browserState
+    NS_DESIGNATED_INITIALIZER;
 
 @end
 
@@ -65,4 +66,15 @@ class ChromeBrowserState;
 
 @end
 
+@interface SessionServiceIOS (Testing)
+
+// 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.
+- (void)performSaveWindow:(SessionWindowIOS*)window
+              toDirectory:(NSString*)directory;
+
+@end
+
 #endif  // IOS_CHROME_BROWSER_SESSIONS_SESSION_SERVICE_H_