[gcb0]

C can already infer that. Google probably does that on their free CDN stuff.

you have unique combination of IP+UserAgent+extra Headers. That is enough. A and B does not even have to send anything. And this will continue to work even without cookies.

[icebraining]

Requiring an IP address already eliminates cross-network tracking. For example, lots of people browse both on their PC on a cable/fiber connection and on their phone/tablet on 3G, with different IPs. They also often browse from their work network (yet another IP).

Same with User Agent: not useful if you're using Chrome on your laptop and Safari on your phone.