Implement SkGLContext swapBuffers with fence syncs

include/gpu/gl/SkNullGLContext.h

 
 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #ifndef SkNullGLContext_DEFINED
 #define SkNullGLContext_DEFINED
 
 #include "gl/SkGLContext.h"
 
 class SK_API SkNullGLContext : public SkGLContext {
 public:
     ~SkNullGLContext() override;
-    void makeCurrent() const override;
-    void swapBuffers() const override {};
 
     static SkNullGLContext* Create(GrGLStandard);
 
     class ContextState;
 
 private:
     SkNullGLContext();
 
+    void onPlatformMakeCurrent() const override;
+    void onPlatformSwapBuffers() const override {}
+    GrGLFuncPtr onPlatformGetProcAddress(const char*) const override { return NULL; }
+
     ContextState* fState;
 };
 
 #endif