[Sync] Refactor ModelSafeWorker::DoWorkAndWaitUntilDone() to avoid code duplication.

components/sync/test/engine/fake_model_worker.h

Index: components/sync/test/engine/fake_model_worker.h
diff --git a/components/sync/test/engine/fake_model_worker.h b/components/sync/test/engine/fake_model_worker.h
index 55ab4a9e9b212d093a1fa125607fd7c0d9b0586b..5801d953671151d24f273e8ffc8bc5a918bf3971 100644
--- a/components/sync/test/engine/fake_model_worker.h
+++ b/components/sync/test/engine/fake_model_worker.h
@@ -22,12 +22,11 @@ class FakeModelWorker : public ModelSafeWorker {
   ModelSafeGroup GetModelSafeGroup() override;
   bool IsOnModelThread() override;
 
- protected:
-  SyncerError DoWorkAndWaitUntilDoneImpl(const WorkCallback& work) override;
-
  private:
   ~FakeModelWorker() override;
 
+  void ScheduleWork(base::Closure work) override;
+
   const ModelSafeGroup group_;
   base::ThreadChecker thread_checker_;