Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/unix/GLWindowContext_unix.h

Index: tools/viewer/sk_app/unix/GLWindowContext_unix.h
diff --git a/tools/viewer/sk_app/unix/GLWindowContext_unix.h b/tools/viewer/sk_app/unix/GLWindowContext_unix.h
deleted file mode 100644
index 2d76254c21012d710557dae0c2f00aff56452305..0000000000000000000000000000000000000000
--- a/tools/viewer/sk_app/unix/GLWindowContext_unix.h
+++ /dev/null
@@ -1,39 +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_unix_DEFINED
-#define GLWindowContext_unix_DEFINED
-
-#include "../GLWindowContext.h"
-#include "Window_unix.h"
-
-namespace sk_app {
-
-class GLWindowContext_unix : public GLWindowContext {
-public:
-    friend GLWindowContext* GLWindowContext::Create(void* platformData, const DisplayParams&);
-
-    ~GLWindowContext_unix() override;
-
-    void onSwapBuffers() override;
-
-    void onInitializeContext(void*, const DisplayParams&) override;
-    void onDestroyContext() override;
-
-private:
-    GLWindowContext_unix(void*, const DisplayParams&);
-
-    Display*     fDisplay;
-    XWindow      fWindow;
-    XVisualInfo* fVisualInfo;
-    GLXContext   fGLContext;
-};
-
-
-}
-
-#endif