Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/RasterWindowContext.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 RasterWindowContext_DEFINED
 #define RasterWindowContext_DEFINED
 
 #include "WindowContext.h"
 
 namespace sk_app {
 
 class RasterWindowContext : public WindowContext {
 public:
-    // This is defined in the platform .cpp file
-    static RasterWindowContext* Create(void* platformData, const DisplayParams& params);
-
     // Explicitly convert nullptr to GrBackendContext is needed for compiling
     GrBackendContext getBackendContext() override { return (GrBackendContext) nullptr; }
 
 protected:
     bool isGpuContext() override { return false; }
 };
 
 }   // namespace sk_app
 
 #endif