Is there a plug-in for Chromium to activate and deactivate Tor?
I've found plug-ins to manage HTTP proxies, but not SOCKS5.
Currently the only safe web browser to use with Tor is Tor Browser. Tor Browser is based on Firefox but incorporates changes which are necessary to protect against profiling attacks. Many of the modifications which are needed are performed by the Torbutton Firefox add-on but other changes have to be made at the code level.
It would be nice if Chrome was an available option, but it would be a large amount of work to audit and perform the necessary changes. It is not sufficient to just enable or disable a SOCKS proxy, but also to remove all options for fingerprinting users.
Privoxy and other HTTP proxies are not safe either because they only can remove fingerprinting information from HTTP traffic. It would be trivial for a malicious website to perform an attack to discover a user's identity by sending the attack traffic over HTTPS and so prevent the HTTP proxy from being able to detect the attempt.
Proxy SwitchySharp supports SOCKS.
Here's an article on How to browse the web anonymously with Google Chrome in Linux that covers configuration in detail.
Note that there are some security holes when using this method to browse via Tor in Chrome, including but possibly/probably not limited to: