Implement BUILD.gn for desktop_capture.

BUG=3441
R=brettw@chromium.org, kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/13809004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6584 4adac7df-926f-26a2-2b94-8c16560cd09d