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_bKGD(png_structp png_ptr, png_infop info_ptr, png_color_16p background)
{
   png_debug1(1, "in %s storage function", "bKGD");

   if (png_ptr == NULL || info_ptr == NULL)
      return;

   png_memcpy(&(info_ptr->background), background, png_sizeof(png_color_16));
   info_ptr->valid |= PNG_INFO_bKGD;
}

png_set_bKGD(png_structp png_ptr, png_infop info_ptr, png_color_16p background)
{
   png_debug1(1, "in %s storage function", "bKGD");

   if (png_ptr == NULL || info_ptr == NULL)
      return;

   png_memcpy(&(info_ptr->background), background, png_sizeof(png_color_16));
   info_ptr->valid |= PNG_INFO_bKGD;
}
