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

components/sync/engine/ui_model_worker.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_SYNC_ENGINE_UI_MODEL_WORKER_H_
 #define COMPONENTS_SYNC_ENGINE_UI_MODEL_WORKER_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/sequence_checker.h"
 #include "base/single_thread_task_runner.h"
+#include "base/synchronization/waitable_event.h"
 #include "components/sync/engine/model_safe_worker.h"
 
 namespace syncer {
 
 // A ModelSafeWorker for UI models (e.g. bookmarks) that
 // accepts work requests from the syncapi that need to be fulfilled
 // from the MessageLoop home to the native model.
 class UIModelWorker : public ModelSafeWorker {
  public:
   explicit UIModelWorker(scoped_refptr<base::SingleThreadTaskRunner> ui_thread);
 
   // ModelSafeWorker implementation.
+  void RequestStop() override;
   ModelSafeGroup GetModelSafeGroup() override;
   bool IsOnModelThread() override;
 
+ protected:
+  SyncerError DoWorkAndWaitUntilDoneImpl(const WorkCallback& work) override;
+
  private:
   ~UIModelWorker() override;
 
-  void ScheduleWork(base::OnceClosure work) override;
-
   // A reference to the UI thread's task runner.
   const scoped_refptr<base::SingleThreadTaskRunner> ui_thread_;
 
+  // Signaled when a task posted by DoWorkAndWaitUntilDoneImpl() is deleted,
+  // i.e. after it runs or when it is abandoned. Reset at the beginning of every
+  // DoWorkAndWaitUntilDoneImpl() call.
+  base::WaitableEvent work_done_or_abandoned_;
+
+  // Signaled from RequestStop(). When this is signaled,
+  // DoWorkAndWaitUntilDoneImpl() returns immediately. This is needed to prevent
+  // the UI thread from joining the sync thread while it is waiting for a
+  // WorkCallback to run on the UI thread. See crbug.com/663600.
+  base::WaitableEvent stop_requested_;
+
+  // Verifies that calls to DoWorkAndWaitUntilDoneImpl() are sequenced.
+  base::SequenceChecker sequence_checker_;
+
   DISALLOW_COPY_AND_ASSIGN(UIModelWorker);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_ENGINE_UI_MODEL_WORKER_H_