Clarified use of process vs processInfo in kill

Modified ```process_utils.scheduleKillTree``` to require the process.pid, as suggested by comment:
https://github.com/wrightt/webpagetest/commit/706b8cceb40e7e589283beac241af913a671ea8d#commitcomment-4066964

Modified variable names to distinguish between "process" (anything with a ".pid") vs "processInfo" (must be a ProcessInfo).