Move SkGLContext and some GrGLInterface implementations to skgputest module

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;
-    static SkNullGLContext* Create();
-
-private:
-    SkNullGLContext();
-
-    void onPlatformMakeCurrent() const override {};
-    void onPlatformSwapBuffers() const override {}
-    GrGLFuncPtr onPlatformGetProcAddress(const char*) const override { return nullptr; }
-};
-
-#endif