Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/mac/Window_mac.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_mac_DEFINED
 #define Window_mac_DEFINED
 
 #include "../Window.h"
 #include "SkChecksum.h"
 #include "SkTDynamicHash.h"
 
 namespace sk_app {
 
-struct ContextPlatformData_mac {
-#if 0
-    // TODO: use Mac-specific objects
-    Display*     fDisplay;
-    XWindow      fWindow;
-    XVisualInfo* fVisualInfo;
-#endif
-};
-
 class Window_mac : public Window {
 public:
     Window_mac() : Window()
 #if 0
                   // TODO: use Mac-specific objects
                   , fDisplay(nullptr)
                   , fWindow(0)
                   , fGC(nullptr)
                   , fVisualInfo(nullptr)
 #endif
@@ skipping 22 matching lines @@
     GC           fGC;
     XVisualInfo* fVisualInfo;
 #endif
     
     int          fMSAASampleCount;
 };
 
 }   // namespace sk_app
 
 #endif