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 c33db91cce89db08192293d209c040c4ec373ce5..b66606f0f9da9b8365875194785c3a668be5bd21 100644
--- a/sync/test/fake_server/fake_server.h
+++ b/sync/test/fake_server/fake_server.h
@@ -57,6 +57,9 @@ class FakeServer {
   // result in successful server operations.
   void InjectEntity(scoped_ptr<FakeServerEntity> entity);
 
+  // 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);
@@ -125,6 +128,8 @@ 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_;
 };