Fix shader compilation error in GrInstancedArraysBench

Change-Id: Ifd87e752fe1712ab4adef6c5f5de8798ab6c4991
Reviewed-on: https://skia-review.googlesource.com/50680
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
diff --git a/bench/GLInstancedArraysBench.cpp b/bench/GLInstancedArraysBench.cpp
index 8a07002..8c00607 100644
--- a/bench/GLInstancedArraysBench.cpp
+++ b/bench/GLInstancedArraysBench.cpp
@@ -143,7 +143,7 @@
         fshaderTxt.append(";\n");
         fsOutName = oFragColor.c_str();
     } else {
-        fsOutName = "gl_FragColor";
+        fsOutName = "sk_FragColor";
     }
 
     fshaderTxt.appendf(