Implement SkGLContext swapBuffers with fence syncs

src/gpu/gl/debug/SkDebugGLContext.h

 
 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #ifndef SkDebugGLContext_DEFINED
 #define SkDebugGLContext_DEFINED
 
 #include "gl/SkGLContext.h"
 
 class SkDebugGLContext : public SkGLContext {
 public:
     ~SkDebugGLContext() override;
-    void makeCurrent() const override {}
-    void swapBuffers() const override {}
 
     static SkDebugGLContext* Create(GrGLStandard forcedGpuAPI) {
         if (kGLES_GrGLStandard == forcedGpuAPI) {
             return NULL;
         }
         return SkNEW(SkDebugGLContext);
     }
 private:
+    void onPlatformMakeCurrent() const override {}
+    void onPlatformSwapBuffers() const override {}
+    GrGLFuncPtr onPlatformGetProcAddress(const char*) const override { return NULL; }
+
     SkDebugGLContext();
 };
 
 #endif