SkBitmap::setPixelRef():  less reference churn

BUG=skia:

Change-Id: I9657e4af5fcc808f9175ff336155374ccc77999d
Reviewed-on: https://skia-review.googlesource.com/5461
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
diff --git a/tests/DrawBitmapRectTest.cpp b/tests/DrawBitmapRectTest.cpp
index 221f0c9..62da1c2 100644
--- a/tests/DrawBitmapRectTest.cpp
+++ b/tests/DrawBitmapRectTest.cpp
@@ -38,7 +38,7 @@
     SkBitmap bm;
     const SkImageInfo info = SkImageInfo::MakeN32Premul(100, 100);
     bm.setInfo(info);
-    bm.setPixelRef(new FailurePixelRef(info), 0, 0)->unref();
+    bm.setPixelRef(sk_make_sp<FailurePixelRef>(info), 0, 0);
     // now our bitmap has a pixelref, but we know it will fail to lock
 
     auto surface(SkSurface::MakeRasterN32Premul(200, 200));