Add steamgames.com for shared-credentials

[rinsuki]

Overall Checklist

• I agree to the project's Developer Certificate of Origin
• The top-level JSON objects are sorted alphabetically
• There are no open pull requests for the same update

for shared-credentials.json

• There's evidence the domains are currently related (SSL certificates, DNS entries, valid links between sites, legal documents etc.)
• If using shared, the new group serves login pages on each of the included domains, and those login pages accept accounts from the others. (For example, we wouldn't use a shared association from google.co.il to google.com, because google.co.il redirects to accounts.google.com for sign in.)
• If using from and to, the new group, the from domain(s) redirect to the to domain to log in.

Evidence:

• https://store.steampowered.com/ footer have a "Steamworks" link, href is http://www.steampowered.com/steamworks/ but redirecting to https://partner.steamgames.com/
• SSL certificates on https://partner.steamgames.com contains https://store.steampowered.com SAN
  • 

[rmondello]

@rinsuki Does steamgames.com serve a login form, or does it just redirect to one of the other domains?

[rinsuki]

@rmondello Yes, https://partner.steamgames.com serving a login form, instead of some cross-domain thing like <iframe>.