Add a flag --log_file which produces the existing behaviour of dumping logs of all severities to a file. By default, warnings and errors will now be output to stderr. This is generally more useful for the testing done with voe_cmd_test. TESTED=logs output to stderr by default and to the usual file when the flag is specified. R=tnakamura@webrtc.org Review URL: https://webrtc-codereview.appspot.com/6849005 git-svn-id: http://webrtc.googlecode.com/svn/trunk@5409 4adac7df-926f-26a2-2b94-8c16560cd09d