Lock Chrome version

Google only keeps the latest Google Chrome version in their apt repository, but this makes it hard for us to keep up. For example, the bump to Chrome v64 crashes our tests due to a known bug: https://gitlab.com/gitlab-org/gitlab-ce/issues/43636#note_60732229

I propose we download the .debs into an S3 bucket and build against those files. I found this mirror here: http://sagasl.uccuyosl.edu.ar/mirror/dl.google.com/linux/chrome/deb/pool/main/g/google-chrome-stable/

We should just sync to our own S3 bucket periodically and pull from the Google repository.