[Sync] Signal UIModelWorker to abort on sync shutdown.

components/sync/driver/glue/ui_model_worker.h

Index: components/sync/driver/glue/ui_model_worker.h
diff --git a/components/sync/driver/glue/ui_model_worker.h b/components/sync/driver/glue/ui_model_worker.h
index c3f76946a1fe0fa0b0e44eab19723e1bc047d1e0..7882f69798d61c101fdcbd13d06ee7ec29781079 100644
--- a/components/sync/driver/glue/ui_model_worker.h
+++ b/components/sync/driver/glue/ui_model_worker.h
@@ -8,6 +8,7 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
+#include "base/synchronization/waitable_event.h"
 #include "components/sync/engine/model_safe_worker.h"
 
 namespace syncer {
@@ -19,7 +20,8 @@ class UIModelWorker : public ModelSafeWorker {
  public:
   explicit UIModelWorker(scoped_refptr<base::SingleThreadTaskRunner> ui_thread);
 
-  // ModelSafeWorker implementation. Called on syncapi SyncerThread.
+  // ModelSafeWorker implementation.
+  void RequestStop() override;
   ModelSafeGroup GetModelSafeGroup() override;
   bool IsOnModelThread() override;
 
@@ -32,6 +34,12 @@ class UIModelWorker : public ModelSafeWorker {
   // A reference to the UI thread's task runner.
   const scoped_refptr<base::SingleThreadTaskRunner> ui_thread_;
 
+  // Signal set when work on native thread is done, aborted due to the thread
+  // being torn down, or aborted due to sync stopping. This is needed because
+  // the UI thread joins the sync thread at shutdown, which creates a deadlock
+  // if the sync thread is waiting on the UI thread. See crbug.com/663600.
+  base::WaitableEvent work_done_or_abandoned_;
+
   DISALLOW_COPY_AND_ASSIGN(UIModelWorker);
 };