GM: replace boilerplate with macros

gm/dcshader.cpp

Index: gm/dcshader.cpp
diff --git a/gm/dcshader.cpp b/gm/dcshader.cpp
index 976e2530ead69789edb0f6efe1161f3b4e0f1327..56265f16c9513ba3ffae361fa5e9f779191cd77b 100644
--- a/gm/dcshader.cpp
+++ b/gm/dcshader.cpp
@@ -231,7 +231,7 @@ protected:
         // This GM exists to test a specific feature of the GPU backend. It does not work with the
         // sw rasterizer, tile modes, etc.
         if (nullptr == canvas->getGrContext()) {
-            this->drawGpuOnlyMessage(canvas);
+            skiagm::GM::DrawGpuOnlyMessage(canvas);
             return;
         }
         SkPaint paint;