I don't recommend starting up and shutting down chromium. I haven't figured out all the memory leaks. However, you can achieve the effect by just leaving the chromium instance running in the background, and show and hide the browser rendering window. This is also a faster way of doing it as you don't incur the speed penalty of having to start up and shut down the browser every time; and chromium doesn't take any CPU when it's not rendering (when you don't run the chromium_step() function).
I've uploaded a demo of how to do this called gm_chromium_1.2_instance_demo.yyz