I can browse to https://safebrowsing.clients.google.com/ but it just redirects me to Google's home page.
iTunes 188.8.131.52, Windows 8.1
I saw this same error on multiple occasions in iTunes 184.108.40.206 on Windows 10 when I tried to sync my iPhone.
I just upgraded to iTunes 12.6, and I no longer see that error.
This is an issue with iTunes 12.5.5 that needs fixing by Apple.