[Sync] Try to fix race conditions in CookieJarMismatch.

components/sync/engine/fake_sync_engine.cc

Index: components/sync/engine/fake_sync_engine.cc
diff --git a/components/sync/engine/fake_sync_engine.cc b/components/sync/engine/fake_sync_engine.cc
index b19d28b08085a73d0dd4cd95b00ea9e729be2ac2..dbc4a74d7f705da170ad0f5e1316178de170fd47 100644
--- a/components/sync/engine/fake_sync_engine.cc
+++ b/components/sync/engine/fake_sync_engine.cc
@@ -97,7 +97,12 @@ void FakeSyncEngine::ClearServerData(
   callback.Run();
 }
 
-void FakeSyncEngine::OnCookieJarChanged(bool account_mismatch, bool empty_jar) {
+void FakeSyncEngine::OnCookieJarChanged(bool account_mismatch,
+                                        bool empty_jar,
+                                        const base::Closure& callback) {
+  if (!callback.is_null()) {
+    callback.Run();
+  }
 }
 
 }  // namespace syncer