This will do the signal conversion for you; it doesn't need a driver and draws its power from the HDMI port. I had to get one like this to connect Mrs. Plug's HDMI laptop to a VGA monitor and it's just plug and play.
What I don't know is how ChromeOS handles mutliple monitors, so your Matrox box may or may not work. But for 8 quid I guess it's worth a punt.
Cheers,
Plug
Originally Posted by Zook
Chrome os is essentially a version of Linux. It is a while since I used one but you can mirror the display on one or more external monitors or use the laptop plus an external display. So if you get a hdmi to vga cable it should work out ok.