Add integraton test for sync backup/rollback.

sync/test/fake_server/fake_server.h

Index: sync/test/fake_server/fake_server.h
diff --git a/sync/test/fake_server/fake_server.h b/sync/test/fake_server/fake_server.h
index 71fdc9cd236ca47e968322269e6071f1edb1ef52..fa1b4d58b3af53e9b9eba4037c4314d8228c075c 100644
--- a/sync/test/fake_server/fake_server.h
+++ b/sync/test/fake_server/fake_server.h
@@ -70,6 +70,9 @@ class FakeServer {
   // authentication error.
   void SetUnauthenticated();
 
+  // Return |error_type| on next sync request.
+  void TriggerError(const sync_pb::SyncEnums::ErrorType& error_type);
+
   // Adds |observer| to FakeServer's observer list. This should be called
   // before the Profile associated with |observer| is connected to the server.
   void AddObserver(Observer* observer);
@@ -142,6 +145,8 @@ class FakeServer {
   // All Keystore keys known to the server.
   std::vector<std::string> keystore_keys_;
 
+  sync_pb::SyncEnums::ErrorType error_type_;
+
   // FakeServer's observers.
   ObserverList<Observer, true> observers_;
 };