fix lint warnings

Author: dconeybe

Firestore/core/src/util/testing_hooks.h

@@ -19,7 +19,7 @@
 
 #include <functional>
 #include <memory>
-#include <mutex>
+#include <mutex>  // NOLINT(build/c++11)
 #include <unordered_map>
 
 #include "Firestore/core/src/api/listener_registration.h"

Firestore/core/test/unit/util/testing_hooks_test.cc

@@ -30,7 +30,7 @@
 
 namespace {
 
-using namespace std::chrono_literals;
+using namespace std::chrono_literals;  // NOLINT(build/namespaces)
 
 using firebase::firestore::api::ListenerRegistration;
 using firebase::firestore::testutil::AsyncTest;