Hi, webshield is not dependent on the browser version/type. It works the same in all browsers. So, I can confirm the correct working of avast web shield in chrome - as you request.
Moreover, there are many ways how could the traces of the virus land on your disk. As Milos suggested, since the detection is very new (2-3 days) it is possible it was already there. What is important is, was is executed by your browser or not?