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/WritePixelsTest.cpp b/tests/WritePixelsTest.cpp
index 2937e0e..1c5ce81 100644
--- a/tests/WritePixelsTest.cpp
+++ b/tests/WritePixelsTest.cpp
@@ -263,8 +263,8 @@
     if (!bm->setInfo(info, rowBytes)) {
         return false;
     }
-    SkPixelRef* pr = SkMallocPixelRef::NewAllocate(info, rowBytes, nullptr);
-    bm->setPixelRef(pr)->unref();
+    sk_sp<SkPixelRef> pr(SkMallocPixelRef::NewAllocate(info, rowBytes, nullptr));
+    bm->setPixelRef(std::move(pr), 0, 0);
     return true;
 }