Create a scanline decoder without creating a codec

src/codec/SkCodec_libpng.h

Index: src/codec/SkCodec_libpng.h
diff --git a/src/codec/SkCodec_libpng.h b/src/codec/SkCodec_libpng.h
index 297e8043fef2e0802739af7011c7d9fdbbc9ef30..21bbdadb4952f458da32abaef0476a4f2da9f921 100644
--- a/src/codec/SkCodec_libpng.h
+++ b/src/codec/SkCodec_libpng.h
@@ -23,18 +23,18 @@ class SkStream;
 
 class SkPngCodec : public SkCodec {
 public:
-    // Assumes IsPng was called and returned true.
-    static SkCodec* NewFromStream(SkStream*);
     static bool IsPng(SkStream*);
 
+    // Assume IsPng was called and returned true.
+    static SkCodec* NewFromStream(SkStream*);
+    static SkScanlineDecoder* NewSDFromStream(SkStream*);
+
     virtual ~SkPngCodec();
 
 protected:
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override { return kPNG_SkEncodedFormat; }
-    SkScanlineDecoder* onGetScanlineDecoder(const SkImageInfo& dstInfo, const Options& options,
-                                            SkPMColor ctable[], int* ctableCount) override;
     bool onReallyHasAlpha() const override { return fReallyHasAlpha; }
 private:
     png_structp                 fPng_ptr;