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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}