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_get_compression_buffer_size(png_structp png_ptr)
{
   return (png_uint_32)(png_ptr? png_ptr->zbuf_size : 0L);
}

png_get_compression_buffer_size(png_structp png_ptr)
{
   return (png_uint_32)(png_ptr? png_ptr->zbuf_size : 0L);
}
