Add SkPngChunkReader.

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 3afaa39bf4f4d84917bf9b3eb7fabaddda6991af..c16c1c18ea7a7543d66a9dae630cccbaf708e68c 100644
--- a/src/codec/SkCodec_libpng.h
+++ b/src/codec/SkCodec_libpng.h
@@ -7,6 +7,7 @@
 
 #include "SkCodec.h"
 #include "SkColorTable.h"
+#include "SkChunkReader.h"
 #include "SkEncodedFormat.h"
 #include "SkImageInfo.h"
 #include "SkRefCnt.h"
@@ -22,7 +23,7 @@ class SkStream;
 class SkPngCodec : public SkCodec {
 public:
     // Assumes IsPng was called and returned true.
-    static SkCodec* NewFromStream(SkStream*);
+    static SkCodec* NewFromStream(SkStream*, SkChunkReader* = NULL);
     static bool IsPng(SkStream*);
 protected:
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*)
@@ -31,6 +32,7 @@ protected:
     SkScanlineDecoder* onGetScanlineDecoder(const SkImageInfo& dstInfo) override;
     bool onReallyHasAlpha() const override { return fReallyHasAlpha; }
 private:
+    SkAutoTUnref<SkChunkReader> fChunkReader;
     png_structp                 fPng_ptr;
     png_infop                   fInfo_ptr;
 
@@ -42,7 +44,7 @@ private:
     int                         fNumberPasses;
     bool                        fReallyHasAlpha;
 
-    SkPngCodec(const SkImageInfo&, SkStream*, png_structp, png_infop);
+    SkPngCodec(const SkImageInfo&, SkStream*, SkChunkReader*, png_structp, png_infop);
     ~SkPngCodec();
 
     // Helper to set up swizzler and color table. Also calls png_read_update_info.