Add script to ensure virtual webcam is running.
Add script to ensure virtual webcam is running.

This script will check that a webcam is running and start it if it's
not currently running.
It's tailored to the way our buildbots are currently configured.

TEST=local execution on Windows, Mac and Linux.
BUG=none
R=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1406005

git-svn-id: 03ae4fbe531b1eefc9d815f31e49022782c42458@3981 4adac7df-926f-26a2-2b94-8c16560cd09d

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