Make SerialIoHandlerWin::UiThreadHelper final.

device/serial/serial_io_handler_win.cc

Index: device/serial/serial_io_handler_win.cc
diff --git a/device/serial/serial_io_handler_win.cc b/device/serial/serial_io_handler_win.cc
index 2a1e752c9f7d7444be2e44ce84c86def4e9ba6b6..d63a34ead2184578a5585f0d1e8f6fd8ccb54b44 100644
--- a/device/serial/serial_io_handler_win.cc
+++ b/device/serial/serial_io_handler_win.cc
@@ -153,7 +153,8 @@ scoped_refptr<SerialIoHandler> SerialIoHandler::Create(
   return new SerialIoHandlerWin(file_thread_task_runner, ui_thread_task_runner);
 }
 
-class SerialIoHandlerWin::UiThreadHelper : public DeviceMonitorWin::Observer {
+class SerialIoHandlerWin::UiThreadHelper final
+    : public DeviceMonitorWin::Observer {
  public:
   UiThreadHelper(
       base::WeakPtr<SerialIoHandlerWin> io_handler,
@@ -368,6 +369,7 @@ SerialIoHandlerWin::SerialIoHandlerWin(
     : SerialIoHandler(file_thread_task_runner, ui_thread_task_runner),
       event_mask_(0),
       is_comm_pending_(false),
+      helper_(nullptr),
       weak_factory_(this) {}
 
 SerialIoHandlerWin::~SerialIoHandlerWin() {