Add OpenGL context to 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 {
 
+// for Windows
+struct ContextPlatformData_win {
+    HINSTANCE fHInstance;
+    HWND      fHWnd;
+};
+
 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, const DisplayParams& params) override;
+    bool attach(BackendType attachType, const DisplayParams& params) override;
 
     void inval() override;
 
 private:
     HINSTANCE fHInstance;
     HWND      fHWnd;
 };
 
 }   // namespace sk_app
 
 #endif