[Sync] Signal UIModelWorker to abort on sync shutdown.

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 caacfcaf47705de59c101d6023f90b6171e1c4b5..29f06cda7f651ec84a0761aa007a226ec4344eef 100644
--- a/components/sync/engine/model_safe_worker.h
+++ b/components/sync/engine/model_safe_worker.h
@@ -67,6 +67,9 @@ class ModelSafeWorker : public base::RefCountedThreadSafe<ModelSafeWorker> {
   // Returns true if called on the thread this worker works on.
   virtual bool IsOnModelThread() = 0;
 
+  // Return true if the worker was stopped. Thread safe.
+  bool IsStopped();
+
  protected:
   ModelSafeWorker();
   virtual ~ModelSafeWorker();
@@ -76,9 +79,6 @@ class ModelSafeWorker : public base::RefCountedThreadSafe<ModelSafeWorker> {
   // from a model-safe thread.
   virtual SyncerError DoWorkAndWaitUntilDoneImpl(const WorkCallback& work) = 0;
 
-  // Return true if the worker was stopped. Thread safe.
-  bool IsStopped();
-
  private:
   friend class base::RefCountedThreadSafe<ModelSafeWorker>;