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

components/history/core/browser/history_model_worker.h

Index: components/history/core/browser/history_model_worker.h
diff --git a/components/history/core/browser/history_model_worker.h b/components/history/core/browser/history_model_worker.h
index 6421c64d3a198d1080cc5ccb00a04d219dbcea03..ca183000c81d494a031463c1823e44e7094f877c 100644
--- a/components/history/core/browser/history_model_worker.h
+++ b/components/history/core/browser/history_model_worker.h
@@ -32,13 +32,11 @@ class HistoryModelWorker : public syncer::ModelSafeWorker {
   syncer::ModelSafeGroup GetModelSafeGroup() override;
   bool IsOnModelThread() override;
 
- protected:
-  syncer::SyncerError DoWorkAndWaitUntilDoneImpl(
-      const syncer::WorkCallback& work) override;
-
  private:
   ~HistoryModelWorker() override;
 
+  void ScheduleWork(base::Closure work) override;

[skym, 2017/04/06 00:34:54]

In the past we've been passing base::Callbacks to functions as const&s. Is there
any point to this? Should we be simply passing them like this? I've never really
understood the difference.

[fdoray, 2017/04/06 18:10:55]

Closure is a simple class that holds a reference to a BindStateBase. Copying a
Closure increments the reference count of this BindStateBase. Moving the Closure
prevents that. 

void ScheduleWork(const base::Closure& work) {
  task_runner->PostTask(FROM_HERE, work); <-- bump ref count
}

vs.

void ScheduleWork(base::Closure work) {
  task_runner->PostTask(FROM_HERE, std::move(work)); <-- ref count unchanged
} 

I'm currently writing documentation about this, so it's normal that you're not
aware of it.

Note: Bumping the ref count costs CPU cycles, but also creates non-determinism
(who will release the last reference?).

+
   const base::WeakPtr<history::HistoryService> history_service_;
 
   // A reference to the UI thread's task runner.