Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/driver/glue/sync_backend_host_impl_unittest.cc

Index: components/sync/driver/glue/sync_backend_host_impl_unittest.cc
diff --git a/components/sync/driver/glue/sync_backend_host_impl_unittest.cc b/components/sync/driver/glue/sync_backend_host_impl_unittest.cc
index 487246eff79e17c3ffb2253ddde405cbe7b8b29f..ea32051bd070f445f202c5a891e35ce1fbb1626b 100644
--- a/components/sync/driver/glue/sync_backend_host_impl_unittest.cc
+++ b/components/sync/driver/glue/sync_backend_host_impl_unittest.cc
@@ -142,11 +142,10 @@ class FakeSyncManagerFactory : public SyncManagerFactory {
 class BackendSyncClient : public FakeSyncClient {
  public:
   scoped_refptr<ModelSafeWorker> CreateModelWorkerForGroup(
-      ModelSafeGroup group,
-      WorkerLoopDestructionObserver* observer) override {
+      ModelSafeGroup group) override {
     switch (group) {
       case GROUP_PASSIVE:
-        return new PassiveModelWorker(observer);
+        return new PassiveModelWorker();
       default:
         return nullptr;
     }