fixed GLVec4ScalarBench

TBR=jvanverth@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2419573002

Review-Url: https://codereview.chromium.org/2419573002
diff --git a/bench/GLVec4ScalarBench.cpp b/bench/GLVec4ScalarBench.cpp
index 373ae6b..8511637 100644
--- a/bench/GLVec4ScalarBench.cpp
+++ b/bench/GLVec4ScalarBench.cpp
@@ -145,7 +145,7 @@
         fshaderTxt.append(";\n");
         fsOutName = oFragColor.c_str();
     } else {
-        fsOutName = "gl_FragColor";
+        fsOutName = "sk_FragColor";
     }