Add SkPngChunkReader.

src/codec/SkCodec_libpng.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodec.h"
 #include "SkColorTable.h"
+#include "SkChunkReader.h"
 #include "SkEncodedFormat.h"
 #include "SkImageInfo.h"
 #include "SkRefCnt.h"
 #include "SkSwizzler.h"
 
 extern "C" {
     #include "png.h"
 }
 
 class SkScanlineDecoder;
 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*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override { return kPNG_SkEncodedFormat; }
     SkScanlineDecoder* onGetScanlineDecoder(const SkImageInfo& dstInfo) override;
     bool onReallyHasAlpha() const override { return fReallyHasAlpha; }
 private:
+    SkAutoTUnref<SkChunkReader> fChunkReader;
     png_structp                 fPng_ptr;
     png_infop                   fInfo_ptr;
 
     // These are stored here so they can be used both by normal decoding and scanline decoding.
     SkAutoTUnref<SkColorTable>  fColorTable;    // May be unpremul.
     SkAutoTDelete<SkSwizzler>   fSwizzler;
 
     SkSwizzler::SrcConfig       fSrcConfig;
     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.
     Result initializeSwizzler(const SkImageInfo& requestedInfo, void* dst,
                               size_t rowBytes, const Options&);
     bool decodePalette(bool premultiply);
     void finish();
 
     friend class SkPngScanlineDecoder;
 
     typedef SkCodec INHERITED;
 };