What steps will reproduce the problem?
(1) Install 'Gerrit UI Switcher' extension
(2) Go to chromium-review.
(3) Turn on extension.
(4) See PolyGerrit UI.
(5) Uninstall 'Gerrit UI Switcher' extension.
(6) Go to chromium-review

What is the expected output?
No longer see PG UI.

What do you see instead?
You see PG UI.

The cookie is still left behind. Might want to look into if you can do steps during extension uninstallation.
Note: Going to *-review.googlesource.com/?polygerrit=0 cleans up the cookie.