Add standard way to indicate GM is GPU-only.

TBR=egdaniel@google.com,mtklein@google.com

Review URL: https://codereview.chromium.org/869393007
diff --git a/gm/yuvtorgbeffect.cpp b/gm/yuvtorgbeffect.cpp
index 1a2c9c9..fb0c67d 100644
--- a/gm/yuvtorgbeffect.cpp
+++ b/gm/yuvtorgbeffect.cpp
@@ -66,6 +66,7 @@
         }
         GrContext* context = rt->getContext();
         if (NULL == context) {
+            this->drawGpuOnlyMessage(canvas);
             return;
         }