[Sync] Signal UIModelWorker to abort on sync shutdown.

components/sync/engine/ui_model_worker.h

Index: components/sync/engine/ui_model_worker.h
diff --git a/components/sync/engine/ui_model_worker.h b/components/sync/engine/ui_model_worker.h
index c158e2a5cbcc14471bf839b1ecdf8101a0541c15..2306a39c52e652a3f9281a175c0b5d0cf0cf0bba 100644
--- a/components/sync/engine/ui_model_worker.h
+++ b/components/sync/engine/ui_model_worker.h
@@ -7,7 +7,9 @@
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/sequence_checker.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 +21,8 @@ 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 +35,20 @@ class UIModelWorker : public ModelSafeWorker {
   // A reference to the UI thread's task runner.
   const scoped_refptr<base::SingleThreadTaskRunner> ui_thread_;
 
+  // Signaled when a task posted by DoWorkAndWaitUntilDoneImpl() is deleted,
+  // i.e. after it runs or when it is abandoned. Reset at the beginning of every
+  // DoWorkAndWaitUntilDoneImpl() call.
+  base::WaitableEvent work_done_or_abandoned_;
+
+  // Signaled from RequestStop(). When this is signaled,
+  // DoWorkAndWaitUntilDoneImpl() returns immediately. This is needed to prevent
+  // the UI thread from joining the sync thread while it is waiting for a
+  // WorkCallback to run on the UI thread. See crbug.com/663600.
+  base::WaitableEvent stop_requested_;
+
+  // Verifies that calls to DoWorkAndWaitUntilDoneImpl() are sequenced.
+  base::SequenceChecker sequence_checker_;
+
   DISALLOW_COPY_AND_ASSIGN(UIModelWorker);
 };