Avoid drawing NVPR DIF text when text size is 0

gm/textblobmixedsizes.cpp

Index: gm/textblobmixedsizes.cpp
diff --git a/gm/textblobmixedsizes.cpp b/gm/textblobmixedsizes.cpp
index 330d33f62cfd603dce4eff6e1fb99397658e3ca9..86ab0ac8fbecb4ab4e2e3ed97a90fe93138f992a 100644
--- a/gm/textblobmixedsizes.cpp
+++ b/gm/textblobmixedsizes.cpp
@@ -73,6 +73,13 @@ protected:
 
         sk_tool_utils::add_to_text_blob(&builder, text, paint, 0, yOffset);
 
+        // Zero size.
+        paint.measureText(text, strlen(text), &bounds);
+        yOffset += bounds.height();
+        paint.setTextSize(0);
+
+        sk_tool_utils::add_to_text_blob(&builder, text, paint, 0, yOffset);
+
         // build
         fBlob.reset(builder.build());
     }