Consolidate SkCodec functions for handling rewind

src/codec/SkCodec_libgif.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 "SkImageInfo.h"
 
@@ skipping 46 matching lines @@
     /*
      * Initiates the gif decode
      */
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&,
             SkPMColor*, int32_t*) override;
 
     SkEncodedFormat onGetEncodedFormat() const override {
         return kGIF_SkEncodedFormat;
     }
 
+    bool onRewind() override;
+
 private:
 
     /*
      * This function cleans up the gif object after the decode completes
      * It is used in a SkAutoTCallIProc template
      */
     static void CloseGif(GifFileType* gif);
 
     /*
      * Frees any extension data used in the decode
      * Used in a SkAutoTCallVProc
      */
     static void FreeExtension(SavedImage* image);
 
     /*
      * Creates an instance of the decoder
      * Called only by NewFromStream
      *
      * @param srcInfo contains the source width and height
      * @param stream the stream of image data
      * @param gif pointer to library type that manages gif decode
      *            takes ownership
      */
     SkGifCodec(const SkImageInfo& srcInfo, SkStream* stream, GifFileType* gif);
 
     SkAutoTCallVProc<GifFileType, CloseGif> fGif; // owned
 
     typedef SkCodec INHERITED;
 };