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/gm.h b/gm/gm.h
index b474729..211e051 100644
--- a/gm/gm.h
+++ b/gm/gm.h
@@ -93,6 +93,9 @@
         }
 
     protected:
+        /** draws a standard message that the GM is only intended to be used with the GPU.*/
+        void drawGpuOnlyMessage(SkCanvas* 
+                                );
         virtual void onOnceBeforeDraw() {}
         virtual void onDraw(SkCanvas*) = 0;
         virtual void onDrawBackground(SkCanvas*);