Apt on SID error with Google-Chrome source

[Stephen Allen <marathon.durandal@gmail.com>]

Lot of changes to Apt recently - this warning has been around a few days
now:

W:
gpgv:/var/lib/apt/lists/dl.google.com_linux_chrome_deb_dists_stable_Release.gpg:
The repository is insufficiently signed by key
4CCA1EAF950CEE4AB83976DCA040830F7FAC5991 (weak digest)

How do I fix?