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

components/sync/engine/browser_thread_model_worker.h

Index: components/sync/engine/browser_thread_model_worker.h
diff --git a/components/sync/engine/browser_thread_model_worker.h b/components/sync/engine/browser_thread_model_worker.h
index 32e63b287d05803e28d21e0d1ed5f3b206277538..3a121f66f6601c32c7c9112a8f29c1c6284b9cbe 100644
--- a/components/sync/engine/browser_thread_model_worker.h
+++ b/components/sync/engine/browser_thread_model_worker.h
@@ -8,6 +8,8 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
+#include "components/sync/base/scoped_event_signal.h"
+#include "components/sync/base/syncer_error.h"
 #include "components/sync/engine/model_safe_worker.h"
 
 namespace syncer {
@@ -26,11 +28,16 @@
   ModelSafeGroup GetModelSafeGroup() override;
   bool IsOnModelThread() override;
 
- private:
+ protected:
   ~BrowserThreadModelWorker() override;
 
-  void ScheduleWork(base::OnceClosure work) override;
+  SyncerError DoWorkAndWaitUntilDoneImpl(const WorkCallback& work) override;
 
+  void CallDoWorkAndSignalTask(const WorkCallback& work,
+                               syncer::ScopedEventSignal scoped_event_signal,
+                               SyncerError* error);
+
+ private:
   scoped_refptr<base::SingleThreadTaskRunner> runner_;
   ModelSafeGroup group_;