Move gflags usage to video_loopback.

gflags aren't used by the test environment and is an unnecessary
dependency. They're only used by the video_loopback target, so moving
them there.

R=mflodman@webrtc.org
BUG=3113

Review URL: https://webrtc-codereview.appspot.com/12379006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6120 4adac7df-926f-26a2-2b94-8c16560cd09d