Do you have a copy of the offline installer for the last working version of Chrome?
I always keep a copy of it for exactly this scenario. Uninstall Chrome, re-install and immediately do all the stuff thats supposed to stop auto-updates - but never does in the longer term. That usually buys me a day or three by which time a patch has hopefully appeared. Chrome always seems to find a way to auto-update eventually.
It's incredibly annoying. Testing is all about control. Chrome does not allow you that.