Clean up test drawContext usage

The general idea is to provide access to SkGpuDevice's drawContext rather than its GrRenderTarget. That is usually what the testing framework actually wants.

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1930623003

Review-Url: https://codereview.chromium.org/1930623003
diff --git a/gm/imagefromyuvtextures.cpp b/gm/imagefromyuvtextures.cpp
index c3184be..a34ec6e 100644
--- a/gm/imagefromyuvtextures.cpp
+++ b/gm/imagefromyuvtextures.cpp
@@ -123,9 +123,8 @@
     }
 
     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;
         }