Make SerialIoHandlerWin::UiThreadHelper final.

Fixes the clang/win build after https://codereview.chromium.org/1439443002/
clang rightfully complains that UiThreadHelper has virtual methods, is
deleted polymorphically, and doesn't have a virtual destructor:

..\..\base/sequenced_task_runner_helpers.h(40,5) :  error: delete called on 'const device::SerialIoHandlerWin::UiThreadHelper' that has virtual functions but non-virtual destructor [-Werror,-Wdelete-non-virtual-dtor]
    delete reinterpret_cast<const T*>(object);
    ^
..\..\base/sequenced_task_runner_helpers.h(86,38) :  note: in instantiation of member function 'base::DeleteHelper<device::SerialIoHandlerWin::UiThreadHelper>::DoDelete' requested here
        from_here, &DeleteHelper<T>::DoDelete, object);
                                     ^
..\..\base/sequenced_task_runner.h(126,48) :  note: in instantiation of function template specialization 'base::subtle::DeleteHelperInternal<device::SerialIoHandlerWin::UiThreadHelper, bool>::DeleteViaSequencedTaskRunner<base::SequencedTaskRunner>' requested here
        subtle::DeleteHelperInternal<T, bool>::DeleteViaSequencedTaskRunner(
                                               ^
..\..\device\serial\serial_io_handler_win.cc(374,28) :  note: in instantiation of function template specialization 'base::SequencedTaskRunner::DeleteSoon<device::SerialIoHandlerWin::UiThreadHelper>' requested here
  ui_thread_task_runner()->DeleteSoon(FROM_HERE, helper_);
                           ^

Making the class final fixes this as it makes sure that nobody adds
a subclass of UiThreadHelper.

Also don't mix initializer styles for the different fields of this
class.

No intended behavior change.

BUG=82385
TBR=juncai

Review URL: https://codereview.chromium.org/1470983002 .

Cr-Commit-Position: refs/heads/master@{#361117}

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() {

device/serial/serial_io_handler_win.h

@@ -66,7 +66,7 @@ class SerialIoHandlerWin : public SerialIoHandler,
 
   // The helper lives on the UI thread and holds a weak reference back to the
   // handler that owns it.
-  UiThreadHelper* helper_ = nullptr;
+  UiThreadHelper* helper_;
   base::WeakPtrFactory<SerialIoHandlerWin> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SerialIoHandlerWin);