Clean up test drawContext usage

gm/imagefromyuvtextures.cpp

Index: gm/imagefromyuvtextures.cpp
diff --git a/gm/imagefromyuvtextures.cpp b/gm/imagefromyuvtextures.cpp
index c3184be1dd23e43b6061a79ca694d36ad63c6428..a34ec6e5138a50c44d798197db35bb751947ada2 100644
--- a/gm/imagefromyuvtextures.cpp
+++ b/gm/imagefromyuvtextures.cpp
@@ -123,9 +123,8 @@ protected:
     }
 
     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;
         }