Fix visual bench memory leak

BUG=skia:

Review URL: https://codereview.chromium.org/1239103002
diff --git a/tools/VisualBench/VisualBench.h b/tools/VisualBench/VisualBench.h
index 683151f..b8cd2bd 100644
--- a/tools/VisualBench/VisualBench.h
+++ b/tools/VisualBench/VisualBench.h
@@ -68,7 +68,7 @@
     WallTimer fTimer;
     State fState;
     SkAutoTDelete<VisualBenchmarkStream> fBenchmarkStream;
-    Benchmark* fBenchmark;
+    SkAutoTUnref<Benchmark> fBenchmark;
 
     // support framework
     SkAutoTUnref<SkSurface> fSurface;