Clean up test drawContext usage

gm/rectangletexture.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 // This test only works with the GPU backend.
 
 #include "gm.h"
@@ skipping 101 matching lines @@
         desc.fOrigin = kTopLeft_GrSurfaceOrigin;
         desc.fTextureHandle = reinterpret_cast<GrBackendObject>(&info);
         if (sk_sp<SkImage> image = SkImage::MakeFromAdoptedTexture(context, desc)) {
             return image;
         }
         GR_GL_CALL(gl, DeleteTextures(1, &id));
         return nullptr;
     }
 
     void onDraw(SkCanvas* canvas) override {
-        GrRenderTarget* rt = canvas->internal_private_accessTopLayerRenderTarget();
-        GrContext* context;
-        if (!rt || !(context = rt->getContext())) {
+        GrContext *context = canvas->getGrContext();
+        if (!context) {
             skiagm::GM::DrawGpuOnlyMessage(canvas);
             return;
         }
 
         static const int kWidth = 50;
         static const int kHeight = 50;
         static const SkScalar kPad = 5.f;
 
         SkPMColor pixels[kWidth * kHeight];
         this->fillPixels(kWidth, kHeight, pixels);
@@ skipping 46 matching lines @@
     }
 
 private:
     typedef GM INHERITED;
 };
 
 DEF_GM(return new RectangleTexture;)
 }
 
 #endif