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_mmx_bitdepth_threshold (png_structp png_ptr)
 {
     /* Obsolete, to be removed from libpng-1.4.0 */
    PNG_UNUSED(png_ptr)
    return 0L;
 }

 png_get_mmx_bitdepth_threshold (png_structp png_ptr)
 {
     /* Obsolete, to be removed from libpng-1.4.0 */
    return (png_ptr? 0: 0);
 }
