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/PDFInvalidBitmapTest.cpp b/tests/PDFInvalidBitmapTest.cpp
index 10292bd..54acf02 100644
--- a/tests/PDFInvalidBitmapTest.cpp
+++ b/tests/PDFInvalidBitmapTest.cpp
@@ -32,7 +32,7 @@
 SkBitmap make_invalid_bitmap(const SkImageInfo& imageInfo) {
     SkBitmap bitmap;
     bitmap.setInfo(imageInfo);
-    bitmap.setPixelRef(new InvalidPixelRef(imageInfo))->unref();
+    bitmap.setPixelRef(sk_make_sp<InvalidPixelRef>(imageInfo), 0 ,0);
     return bitmap;
 }