Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/win/GLWindowContext_win.h

Index: tools/viewer/sk_app/win/GLWindowContext_win.h
diff --git a/tools/viewer/sk_app/win/GLWindowContext_win.h b/tools/viewer/sk_app/win/GLWindowContext_win.h
deleted file mode 100644
index f30a805e7234a4374216ce3f96643bb53f5c26a1..0000000000000000000000000000000000000000
--- a/tools/viewer/sk_app/win/GLWindowContext_win.h
+++ /dev/null
@@ -1,37 +0,0 @@
-
-/*
- * 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 GLWindowContext_win_DEFINED
-#define GLWindowContext_win_DEFINED
-
-#include <windows.h>
-#include "../GLWindowContext.h"
-
-namespace sk_app {
-
-class GLWindowContext_win : public GLWindowContext {
-public:
-    friend GLWindowContext* GLWindowContext::Create(void* platformData, const DisplayParams&);
-
-    ~GLWindowContext_win() override;
-
-    void onSwapBuffers() override;
-
-    void onInitializeContext(void*, const DisplayParams&) override;
-    void onDestroyContext() override;
-
-private:
-    GLWindowContext_win(void*, const DisplayParams&);
-
-    HWND              fHWND;
-    HGLRC             fHGLRC;
-};
-
-
-}
-
-#endif