[Sync] Replace manual DI with SetTestingFactory.

components/ukm/observers/sync_disable_observer_unittest.cc

Index: components/ukm/observers/sync_disable_observer_unittest.cc
diff --git a/components/ukm/observers/sync_disable_observer_unittest.cc b/components/ukm/observers/sync_disable_observer_unittest.cc
index 4e4080e38fb002f4ffad4a31675c292f1364af55..05f9f2f7fad63cb346b7ee7a3dc88c27c097ddd3 100644
--- a/components/ukm/observers/sync_disable_observer_unittest.cc
+++ b/components/ukm/observers/sync_disable_observer_unittest.cc
@@ -28,7 +28,7 @@ class MockSyncService : public syncer::FakeSyncService {
     }
   }
 
-  void Shutdown() {
+  void Shutdown() override {
     for (auto& observer : observers_) {
       observer.OnSyncShutdown(this);
     }