Revert "Sync: Avoid deadlock in SyncBackendRegistrar / ModelSafeWorker on sync backend shutdown."

sync/internal_api/public/engine/model_safe_worker.h

Index: sync/internal_api/public/engine/model_safe_worker.h
diff --git a/sync/internal_api/public/engine/model_safe_worker.h b/sync/internal_api/public/engine/model_safe_worker.h
index df5bba9b7b10109863c29c281aeaf8575e0f67b5..02f3ca1fa2b18f8ce100ffedbda7342b5edc2e99 100644
--- a/sync/internal_api/public/engine/model_safe_worker.h
+++ b/sync/internal_api/public/engine/model_safe_worker.h
@@ -133,13 +133,7 @@ class SYNC_EXPORT ModelSafeWorker
   // observation from sync thread when shutting down sync.
   base::Lock working_loop_lock_;
   base::MessageLoop* working_loop_;
-
-  // Callback passed with UnregisterForLoopDestruction. Normally this
-  // remains unset/unused and is stored only if |working_loop_| 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_;
+  base::WaitableEvent working_loop_set_wait_;
 };
 
 // A map that details which ModelSafeGroup each ModelType