Make sp variants for SkData

Review URL: https://codereview.chromium.org/1779263003
diff --git a/src/images/SkImageDecoder_libbmp.cpp b/src/images/SkImageDecoder_libbmp.cpp
index 4a6f71c..b9359be 100644
--- a/src/images/SkImageDecoder_libbmp.cpp
+++ b/src/images/SkImageDecoder_libbmp.cpp
@@ -97,7 +97,7 @@
     // First read the entire stream, so that all of the data can be passed to
     // the BmpDecoderHelper.
 
-    SkAutoTUnref<SkData> data(SkCopyStreamToData(stream));
+    auto data = SkCopyStreamToData(stream);
     if (!data) {
         return kFailure;
     }