Chromium Nightly Build Downloader
I assume you are aware of Chrome Canary, which I gather is able to use binary deltas? I don't think it's pure Chromium though.

Your code is pretty clean. You could be using argparse for parsing the command line, currently you are ignoring flags that don't match. You could wrap the exit call around main, like this: exit(main()), and use return / return 2 statements instead. You could replace "C:\\Program Files" with %PROGRAMFILES%, or pick up some defaults from the registry, but that's not necessary unless you plan for a larger audience. You could also reduce the window Chromium is unavailable by unzipping somewhere else, then moving the directories, then clearing the previous one (but I don't think you can make it atomic without something like symlinks or hardlinks).