[Sync] Move thread checking into the ModelSafeWorker interface.

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 56c75fe0a1dbab7b652032152ecee0f07390cbd7..e44a916f315cd8144d7abeb5b464d0cdf8f34a77 100644
--- a/components/sync/driver/glue/sync_backend_host_impl_unittest.cc
+++ b/components/sync/driver/glue/sync_backend_host_impl_unittest.cc
@@ -213,8 +213,7 @@ class SyncBackendHostTest : public testing::Test {
                        base::Unretained(network_resources_.get()), nullptr,
                        base::Bind(&EmptyNetworkTimeUpdate));
     backend_->Initialize(
-        &mock_frontend_, &sync_thread_, base::ThreadTaskRunnerHandle::Get(),
-        base::ThreadTaskRunnerHandle::Get(), WeakHandle<JsEventHandler>(),
+        &mock_frontend_, &sync_thread_, WeakHandle<JsEventHandler>(),
         GURL(std::string()), std::string(), credentials_, true, false,
         base::FilePath(), std::move(fake_manager_factory_),
         MakeWeakHandle(test_unrecoverable_error_handler_.GetWeakPtr()),