Cleanup GrContextFactory and make it's subclasses private

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 "SkGLContext.h"
-
-class SK_API SkNullGLContext : public SkGLContext {
-public:
-    virtual ~SkNullGLContext() SK_OVERRIDE;
-    virtual void makeCurrent() const SK_OVERRIDE {};
-    virtual void swapBuffers() const SK_OVERRIDE {};
-
-    static SkNullGLContext* Create(GrGLStandard forcedGpuAPI) {
-        if (kGLES_GrGLStandard == forcedGpuAPI) {
-            return NULL;
-        }
-        SkNullGLContext* ctx = SkNEW(SkNullGLContext);
-        if (!ctx->isValid()) {
-            SkDELETE(ctx);
-            return NULL;
-        }
-        return ctx;
-    }
-
-private:
-    SkNullGLContext();
-};
-
-#endif