third_party/libpng: update to 1.2.54

TBR=darin@chromium.org
BUG=560291

Review URL: https://codereview.chromium.org/1467263003

Cr-Commit-Position: refs/heads/master@{#362298}
png_permit_mng_features (png_structp png_ptr, png_uint_32 mng_features)
{
   png_debug(1, "in png_permit_mng_features");

   if (png_ptr == NULL)
      return (png_uint_32)0;
   png_ptr->mng_features_permitted =
     (png_byte)(mng_features & PNG_ALL_MNG_FEATURES);
   return (png_uint_32)png_ptr->mng_features_permitted;
}

png_permit_mng_features (png_structp png_ptr, png_uint_32 mng_features)
{
   png_debug(1, "in png_permit_mng_features");

   if (png_ptr == NULL)
      return (png_uint_32)0;
   png_ptr->mng_features_permitted =
     (png_byte)(mng_features & PNG_ALL_MNG_FEATURES);
   return (png_uint_32)png_ptr->mng_features_permitted;
}
