SkCodec::onGetScanlineDecoder must call rewind.

include/codec/SkCodec.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 SkCodec_DEFINED
 #define SkCodec_DEFINED
 
@@ skipping 103 matching lines @@
 #else
         return this->getInfo().dimensions();
 #endif
     }
 
     virtual SkEncodedFormat onGetEncodedFormat() const = 0;
 
     /**
      *  Override if your codec supports scanline decoding.
      *
-     *  No need to call rewindIfNeeded(), which will have already been called
-     *  by the base class.
+     *  As in onGetPixels(), the implementation must call rewindIfNeeded() and
+     *  handle as appropriate.
      *
      *  @param dstInfo Info of the destination. If the dimensions do not match
      *      those of getInfo, this implies a scale.
      *  @return New SkScanlineDecoder on success, NULL otherwise. The SkCodec
      *      will take ownership of the returned scanline decoder.
      */
     virtual SkScanlineDecoder* onGetScanlineDecoder(const SkImageInfo& dstInfo) {
         return NULL;
     }
 
@@ skipping 33 matching lines @@
 #ifdef SK_SUPPORT_LEGACY_BOOL_ONGETINFO
     const SkImageInfo                   fInfo;
 #endif  //  SK_SUPPORT_LEGACY_BOOL_ONGETINFO
     SkAutoTDelete<SkStream>             fStream;
     bool                                fNeedsRewind;
     SkAutoTDelete<SkScanlineDecoder>    fScanlineDecoder;
 
     typedef SkImageGenerator INHERITED;
 };
 #endif // SkCodec_DEFINED