GM: replace boilerplate with macros

gm/textblobrandomfont.cpp

Index: gm/textblobrandomfont.cpp
diff --git a/gm/textblobrandomfont.cpp b/gm/textblobrandomfont.cpp
index 862d83b7079a6e0bbe084f01760c27bda8ac169a..f21380ee381aed2286d535bc3303b23b1c374109 100644
--- a/gm/textblobrandomfont.cpp
+++ b/gm/textblobrandomfont.cpp
@@ -94,7 +94,7 @@ 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;
         }