GM: replace boilerplate with macros

gm/textblobuseaftergpufree.cpp

Index: gm/textblobuseaftergpufree.cpp
diff --git a/gm/textblobuseaftergpufree.cpp b/gm/textblobuseaftergpufree.cpp
index 91681b82ac6404b83825a49b59cd4268f22b835f..61df533cf78aa402f9fc2bef3d231c679b524a50 100644
--- a/gm/textblobuseaftergpufree.cpp
+++ b/gm/textblobuseaftergpufree.cpp
@@ -32,7 +32,7 @@ protected:
     void onDraw(SkCanvas* canvas) override {
         // This GM exists to test a specific feature of the GPU backend.
         if (nullptr == canvas->getGrContext()) {
-            this->drawGpuOnlyMessage(canvas);
+            skiagm::GM::DrawGpuOnlyMessage(canvas);
             return;
         }