Revert of Delete SkImageDecoder

include/core/SkPngChunkReader.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.
  */
 
 #ifndef SkPngChunkReader_DEFINED
 #define SkPngChunkReader_DEFINED
 
 #include "SkTypes.h"
 #include "SkRefCnt.h"
 
 /**
  *  SkPngChunkReader
  *
  *  Base class for optional callbacks to retrieve meta/chunk data out of a PNG
  *  encoded image as it is being decoded.
- *  Used by SkCodec.
+ *  Used by SkImageDecoder and SkCodec.
  */
 class SkPngChunkReader : public SkRefCnt {
 public:
     /**
      *  This will be called by the decoder when it sees an unknown chunk.
      *
      *  Use by SkCodec:
      *  Depending on the location of the unknown chunks, this callback may be
      *  called by
      *      - the factory (NewFromStream/NewFromData)
      *      - getPixels
      *      - startScanlineDecode
      *      - the first call to getScanlines/skipScanlines
      *  The callback may be called from a different thread (e.g. if the SkCodec
      *  is passed to another thread), and it may be called multiple times, if
      *  the SkCodec is used multiple times.
      *
      *  @param tag Name for this type of chunk.
      *  @param data Data to be interpreted by the subclass.
      *  @param length Number of bytes of data in the chunk.
      *  @return true to continue decoding, or false to indicate an error, which
      *      will cause the decoder to not return the image.
      */
     virtual bool readChunk(const char tag[], const void* data, size_t length) = 0;
 };
 #endif // SkPngChunkReader_DEFINED