* Only allows setting the log file at runtime if not set by an Env Var
* LogModulePrefWatcher will watch logging.config. prefs and call into Logging.cpp to set the log file, sync or timestamp
* Log files set by pref will have a -main.PID or -child.PID suffix
* If the logging.config.clear_on_startup pref is true, prefs will be reset upon restart.
MozReview-Commit-ID: CWJujX4jm2A