Exit after printing usage message.

We should not continue the program if the user asked for help.

Tested on Linux with the following command line:

$ out/Debug/frame_analyzer --help

BUG=None
TEST=see above
R=kjellander@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44069004

Patch from Thiago Farina <tfarina@chromium.org>.

Cr-Commit-Position: refs/heads/master@{#8961}
5 files changed