Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/engine/model_safe_worker.h

Index: components/sync/engine/model_safe_worker.h
diff --git a/components/sync/engine/model_safe_worker.h b/components/sync/engine/model_safe_worker.h
index 2a075ac80c0fcc1248c91e59774b1b0802df983b..869bbae47db3afecc6113bb56688495164663f60 100644
--- a/components/sync/engine/model_safe_worker.h
+++ b/components/sync/engine/model_safe_worker.h
@@ -8,13 +8,11 @@
 #include <map>
 #include <memory>
 #include <string>
-#include <vector>
 
-#include "base/callback.h"
+#include "base/callback_forward.h"
+#include "base/macros.h"
 #include "base/memory/ref_counted.h"
-#include "base/message_loop/message_loop.h"
-#include "base/single_thread_task_runner.h"
-#include "base/synchronization/lock.h"
+#include "base/synchronization/atomic_flag.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/base/syncer_error.h"
 
@@ -46,13 +44,6 @@ enum ModelSafeGroup {
 
 std::string ModelSafeGroupToString(ModelSafeGroup group);
 
-// WorkerLoopDestructionObserver is notified when the thread where it works
-// is going to be destroyed.
-class WorkerLoopDestructionObserver {
- public:
-  virtual void OnWorkerLoopDestroyed(ModelSafeGroup group) = 0;
-};
-
 // The Syncer uses a ModelSafeWorker for all tasks that could potentially
 // modify syncable entries (e.g under a WriteTransaction). The ModelSafeWorker
 // only knows how to do one thing, and that is take some work (in a fully
@@ -60,23 +51,9 @@ class WorkerLoopDestructionObserver {
 // is guaranteed to be "model-safe", where "safe" refers to not allowing us to
 // cause an embedding application model to fall out of sync with the
 // syncable::Directory due to a race. Each ModelSafeWorker is affiliated with
-// a thread and does actual work on that thread. On the destruction of that
-// thread, the affiliated worker is effectively disabled to do more
-// work and will notify its observer.
-class ModelSafeWorker : public base::RefCountedThreadSafe<ModelSafeWorker>,
-                        public base::MessageLoop::DestructionObserver {
+// a thread and does actual work on that thread.
+class ModelSafeWorker : public base::RefCountedThreadSafe<ModelSafeWorker> {
  public:
-  // Subclass should implement to observe destruction of the loop where
-  // it actually does work. Called on UI thread immediately after worker is
-  // created.
-  virtual void RegisterForLoopDestruction() = 0;
-
-  // Called on sync loop from SyncBackendRegistrar::ShutDown(). Post task to
-  // working loop to stop observing loop destruction and invoke
-  // |unregister_done_callback|.
-  virtual void UnregisterForLoopDestruction(
-      base::Callback<void(ModelSafeGroup)> unregister_done_callback);
-
   // If not stopped, call DoWorkAndWaitUntilDoneImpl() to do work. Otherwise
   // return CANNOT_DO_WORK.
   SyncerError DoWorkAndWaitUntilDone(const WorkCallback& work);
@@ -87,12 +64,9 @@ class ModelSafeWorker : public base::RefCountedThreadSafe<ModelSafeWorker>,
 
   virtual ModelSafeGroup GetModelSafeGroup() = 0;
 
-  // MessageLoop::DestructionObserver implementation.
-  void WillDestroyCurrentMessageLoop() override;
-
  protected:
-  explicit ModelSafeWorker(WorkerLoopDestructionObserver* observer);
-  ~ModelSafeWorker() override;
+  ModelSafeWorker();
+  virtual ~ModelSafeWorker();
 
   // Any time the Syncer performs model modifications (e.g employing a
   // WriteTransaction), it should be done by this method to ensure it is done
@@ -102,35 +76,13 @@ class ModelSafeWorker : public base::RefCountedThreadSafe<ModelSafeWorker>,
   // Return true if the worker was stopped. Thread safe.
   bool IsStopped();
 
-  // Subclass should call this in RegisterForLoopDestruction() from the loop
-  // where work is done.
-  void SetWorkingLoopToCurrent();
-
  private:
   friend class base::RefCountedThreadSafe<ModelSafeWorker>;
 
-  void UnregisterForLoopDestructionAsync(
-      base::Callback<void(ModelSafeGroup)> unregister_done_callback);
-
-  // Whether the worker should/can do more work. Set when sync is disabled or
-  // when the worker's working thread is to be destroyed.
-  base::Lock stopped_lock_;
-  bool stopped_;
-
-  // Notified when working thread of the worker is to be destroyed.
-  WorkerLoopDestructionObserver* observer_;
-
-  // Remember working loop for posting task to unregister destruction
-  // observation from sync thread when shutting down sync.
-  base::Lock working_task_runner_lock_;
-  scoped_refptr<base::SingleThreadTaskRunner> working_task_runner_;
+  // Whether the worker should do more work. Set when sync is disabled.
+  base::AtomicFlag stopped_;
 
-  // Callback passed with UnregisterForLoopDestruction. Normally this
-  // remains unset/unused and is stored only if |working_task_runner_| isn't
-  // initialized by the time UnregisterForLoopDestruction is called.
-  // It is safe to copy and thread safe.
-  // See comments in model_safe_worker.cc for more details.
-  base::Callback<void(ModelSafeGroup)> unregister_done_callback_;
+  DISALLOW_COPY_AND_ASSIGN(ModelSafeWorker);
 };
 
 // A map that details which ModelSafeGroup each ModelType