Consolidate SkCodec functions for handling rewind

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 17 matching lines @@
     // Assume IsPng was called and returned true.
     static SkCodec* NewFromStream(SkStream*);
     static SkScanlineDecoder* NewSDFromStream(SkStream*);
 
     virtual ~SkPngCodec();
 
 protected:
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override { return kPNG_SkEncodedFormat; }
+    bool onRewind() override;
     bool onReallyHasAlpha() const override { return fReallyHasAlpha; }
 private:
     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;
     int                         fBitDepth;
 
     SkPngCodec(const SkImageInfo&, SkStream*, png_structp, png_infop, int);
 
 
     // Helper to set up swizzler and color table. Also calls png_read_update_info.
     Result initializeSwizzler(const SkImageInfo& requestedInfo, const Options&,
                               SkPMColor*, int* ctableCount);
 
-    // Calls rewindIfNeeded and returns true if the decoder can continue.
-    bool handleRewind();
     bool decodePalette(bool premultiply, int* ctableCount);
     void destroyReadStruct();
 
     friend class SkPngScanlineDecoder;
     friend class SkPngInterlacedScanlineDecoder;
 
     typedef SkCodec INHERITED;
 };