Enable backend switching for Windows Viewer

tools/viewer/sk_app/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 {}
+    ~Window_win() override { DestroyWindow(fHWnd); }
 
     bool init(HINSTANCE instance);
 
     void setTitle(const char*) override;
     void show() override;
 
     bool attach(BackendType attachType, const DisplayParams& params) override;
 
     void onInval() override;
 
 private:
     HINSTANCE fHInstance;
     HWND      fHWnd;
 };
 
 }   // namespace sk_app
 
 #endif