Steps for Mac and Windows

This change contains a few things:
a) It removes xvfb-run from the mac steps. I've got the mac configured and working running headless tests without xvfb -- all that's necessary is "grunt test-buildbot" now. The same should be true for windows in the very near future.
b) It removes the dependency on an external script. While calling out to a script is all well and good, for a process this simple it shouldn't be necessary. All the logic necessary should be able to be contained in gen_steps or grunt. FWIW, explicitly catching the return value of grunt and printing STEP_FAILURE isn't necessary, as any non-zero return value results in STEP_FAILURE.