I too am a victim of Chrome being installed during an Avast Update with no Dialogue Box appearing to allow me to Untick the Chrome Install. But I figured out a solution. Try this, it worked for me.
Solution: Allow Avast to Update the Program as per normal. Avast will ask to reboot your Computer for the changes to take effect. Allow it, BUT, during the reboot, disconnect your Computer from the Internet. In my case, I simply switched off my ADSL Modem. Now when Windows started, low and behold, the errant Dialogue Box appeared on the Screen. I Unticked the Boxes related to Chrome and let Avast finish. Wait until the Avast Icon has appeared in the bottom right of your Taskbar (you know, where your other start up programs are), before connecting your Computer back to the Internet. Hope this works for you.
Yes, you should not have to go through all this but since Avast will not fix the problem with this Dialogue Box not appearing under normal circumstances, they have left the Users with no choice but to find a solution themselves.
Regards