[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..7165621318f77916f676a9181b4bf3acac8e992e 100644
--- a/components/sync/engine/model_safe_worker.h
+++ b/components/sync/engine/model_safe_worker.h
@@ -62,6 +62,9 @@ class ModelSafeWorker : public base::RefCountedThreadSafe<ModelSafeWorker> {
   // or browser is shutting down. Called on UI loop.
   virtual void RequestStop();
 
+  // Return true if the worker was stopped. Thread safe.
+  bool IsStopped();
+
   virtual ModelSafeGroup GetModelSafeGroup() = 0;
 
   // Returns true if called on the thread this worker works on.
@@ -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>;