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_set_tRNS_to_alpha(png_structp png_ptr)
{
   png_debug(1, "in png_set_tRNS_to_alpha");

   png_ptr->transformations |= (PNG_EXPAND | PNG_EXPAND_tRNS);
   png_ptr->flags &= ~PNG_FLAG_ROW_INIT;
}

png_set_tRNS_to_alpha(png_structp png_ptr)
{
   png_debug(1, "in png_set_tRNS_to_alpha");

   png_ptr->transformations |= (PNG_EXPAND | PNG_EXPAND_tRNS);
   png_ptr->flags &= ~PNG_FLAG_ROW_INIT;
}
