Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/unix/Window_unix.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_unix_DEFINED
 #define Window_unix_DEFINED
 
 #include <X11/Xlib.h>
 #include <GL/glx.h>
 #include "../Window.h"
 #include "SkChecksum.h"
 #include "SkTDynamicHash.h"
 
 typedef Window XWindow;
 
 namespace sk_app {
 
-struct ContextPlatformData_unix {
-    Display*     fDisplay;
-    XWindow      fWindow;
-    XVisualInfo* fVisualInfo;
-};
-
 class Window_unix : public Window {
 public:
     Window_unix() : Window()
                   , fDisplay(nullptr)
                   , fWindow(0)
                   , fGC(nullptr)
                   , fVisualInfo(nullptr)
                   , fMSAASampleCount(0) {}
     ~Window_unix() override { this->closeWindow(); }
 
@@ skipping 51 matching lines @@
 
     bool     fPendingPaint;
     int      fPendingWidth;
     int      fPendingHeight;
     bool     fPendingResize;
 };
 
 }   // namespace sk_app
 
 #endif