Fix ScreenCapturerLinux not to use XDamage when requested.

When moving this code to webrtc I added line "use_x_damage=true" for
debugging and forgot to remove it when landing this code, so the
capturer always tries to use XDamage.

BUG=crbug.com/263003
R=wez@chromium.org

Review URL: https://webrtc-codereview.appspot.com/1854004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@4387 4adac7df-926f-26a2-2b94-8c16560cd09d