gn: Hide modules/video_capture:video_capture_internal_impl behind an arg

R=andresp@webrtc.org, brettw@chromium.org, kjellander@webrtc.org, pbos@webrtc.org, brettw

Review URL: https://webrtc-codereview.appspot.com/30479004

Patch from Cem Kocagil <ckocagil@chromium.org>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7274 4adac7df-926f-26a2-2b94-8c16560cd09d