Clean up test drawContext usage

gm/imagefromyuvtextures.cpp

 /*
  * Copyright 2015 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 105 matching lines @@
         }
 
         for (int i = 0; i < 3; ++i) {
             gpu->deleteTestingOnlyBackendTexture(yuvHandles[i]);
         }
 
         context->resetContext();
     }
 
     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;
         }
 
         GrBackendObject yuvHandles[3];
         this->createYUVTextures(context, yuvHandles);
 
         static const SkScalar kPad = 10.f;
 
         SkISize sizes[] = {
@@ skipping 24 matching lines @@
 
     static const int kBmpSize = 32;
 
     typedef GM INHERITED;
 };
 
 DEF_GM(return new ImageFromYUVTextures;)
 }
 
 #endif