Move all knowledge of X sampling into SkScaledCodec

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 8ef1ae2c17d3ed2e39f64cce2032f868b4f9030e..3a16cc6813ed4a0dc9553d86c3e5c6ac16efbc79 100644
--- a/src/codec/SkCodec_libpng.h
+++ b/src/codec/SkCodec_libpng.h
@@ -41,6 +41,7 @@ protected:
     // Helper to set up swizzler and color table. Also calls png_read_update_info.
     Result initializeSwizzler(const SkImageInfo& requestedInfo, const Options&,
                               SkPMColor*, int* ctableCount);
+    SkSampler* getSampler() override { return fSwizzler; }
 
     SkPngCodec(const SkImageInfo&, SkStream*, png_structp, png_infop, int, int);