Fill incomplete images in SkCodec parent class

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 e8807499ea74419a7e81535a01c44af589f24466..e724dad09ef46305e405c244cb507db9e0522453 100644
--- a/src/codec/SkCodec_libpng.h
+++ b/src/codec/SkCodec_libpng.h
@@ -37,6 +37,7 @@ protected:
     SkEncodedFormat onGetEncodedFormat() const override { return kPNG_SkEncodedFormat; }
     bool onRewind() override;
     bool onReallyHasAlpha() const override { return fReallyHasAlpha; }
+    uint32_t onGetFillValue(const SkImageInfo& dstInfo) const override;
 
 private:
     png_structp                 fPng_ptr;
@@ -46,6 +47,7 @@ private:
     SkAutoTUnref<SkColorTable>  fColorTable;    // May be unpremul.
     SkAutoTDelete<SkSwizzler>   fSwizzler;
 
+    int                         fRow;
     SkSwizzler::SrcConfig       fSrcConfig;
     int                         fNumberPasses;
     bool                        fReallyHasAlpha;