Enable backend switching for Windows Viewer

tools/viewer/sk_app/Window.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_DEFINED
 #define Window_DEFINED
 
 #include "DisplayParams.h"
 #include "SkRect.h"
 #include "SkTouchGesture.h"
 #include "SkTypes.h"
 #include "SkJSONCPP.h"
 
 class SkCanvas;
 class SkSurface;
 
 namespace sk_app {
 
 class WindowContext;
 
 class Window {
 public:
     static Window* CreateNativeWindow(void* platformData);
 
-    virtual ~Window() {};
+    virtual ~Window() { this->detach(); };
 
     virtual void setTitle(const char*) = 0;
     virtual void show() = 0;
     virtual void setUIState(const Json::Value& state) {}  // do nothing in default
 
     // Shedules an invalidation event for window if one is not currently pending.
     // Make sure that either onPaint or markInvalReceived is called when the client window consumes
     // the the inval event. They unset fIsContentInvalided which allow future onInval.
     void inval();
 
@@ skipping 153 matching lines @@
     virtual void onInval() = 0;
 
     // Uncheck fIsContentInvalided to allow future inval/onInval.
     void markInvalProcessed();
 
     bool fIsContentInvalidated = false;  // use this to avoid duplicate invalidate events
 };
 
 }   // namespace sk_app
 #endif