More refactoring for Viewer

tools/viewer/win/Window_win.h

-/*
-* Copyright 2016 Google Inc.
-*
-* Use of this source code is governed by a BSD-style license that can be
-* found in the LICENSE file.
-*/
-
-#ifndef Window_win_DEFINED
-#define Window_win_DEFINED
-
-#include <windows.h>
-#include "../Window.h"
-
-namespace sk_app {
-
-class Window_win : public Window {
-public:
-    Window_win() : Window() {}
-    ~Window_win() override {}
-
-    bool init(HINSTANCE instance);
-
-    void setTitle(const char*) override;
-    void show() override;
-
-    bool attach(BackEndType attachType, int msaaSampleCount) override;
-
-    void inval() override;
-
-private:
-    HINSTANCE fHInstance;
-    HWND      fHWnd;
-};
-
-}   // namespace sk_app
-
-#endif