Fix leak in PictureNestingBench

R=mtklein@google.com
TBR=mtklein@google.com

Author: fmalita@chromium.org

Review URL: https://codereview.chromium.org/569313004
diff --git a/bench/PictureNestingBench.cpp b/bench/PictureNestingBench.cpp
index aecd7ed..f1adb60 100644
--- a/bench/PictureNestingBench.cpp
+++ b/bench/PictureNestingBench.cpp
@@ -23,8 +23,8 @@
         fPaint.setColor(SK_ColorRED);
         fPaint.setAntiAlias(true);
         fPaint.setStyle(SkPaint::kStroke_Style);
-        fName.printf("picture_nesting_%s_%d", name,
-                     this->sierpinsky(SkCreateNullCanvas(), 0, fPaint));
+        SkAutoTUnref<SkCanvas> nullCanvas(SkCreateNullCanvas());
+        fName.printf("picture_nesting_%s_%d", name, this->sierpinsky(nullCanvas, 0, fPaint));
     }
 
 protected: