Fix memory leak in nanobench

This is cleanup for (Add MultiPictureDraw to nanobench - https://codereview.chromium.org/731973005/)

Review URL: https://codereview.chromium.org/730343003

bench/SKPBench.cpp

@@ -73,7 +73,8 @@ void SKPBench::onPerCanvasPostDraw(SkCanvas* canvas) {
     // Draw the last set of tiles into the master canvas in case we're
     // saving the images
     for (int i = 0; i < fTileRects.count(); ++i) {
-        canvas->drawImage(fSurfaces[i]->newImageSnapshot(),
+        SkAutoTUnref<SkImage> image(fSurfaces[i]->newImageSnapshot());
+        canvas->drawImage(image,
                           SkIntToScalar(fTileRects[i].fLeft), SkIntToScalar(fTileRects[i].fTop));
         SkSafeSetNull(fSurfaces[i]);
     }