Rename linuxwindowpicker to x11windowpicker & only use it with use_x11

These days we have Linux chromium builds that don't use X11. We don't
want webrtc to add an X11 dependency to those builds.

BUG=3625
R=henrike@webrtc.org, tnakamura@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/20049004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6909 4adac7df-926f-26a2-2b94-8c16560cd09d