Make SkPngCodec support rewinding properly.

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 "SkEncodedFormat.h"
@@ skipping 32 matching lines @@
     bool                        fReallyHasAlpha;
 
     SkPngCodec(const SkImageInfo&, SkStream*, 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();
+    void destroyReadStruct();
 
     friend class SkPngScanlineDecoder;
 
     typedef SkCodec INHERITED;
 };