Add DesktopCapturer interface for desktop capturers.

The new DesktopCapturer interface will be used for screen and window
captures. Beside DesktopCapturer itself also added classes/interfaces
that it depends on.

R=alexeypa@chromium.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1322007

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@3917 4adac7df-926f-26a2-2b94-8c16560cd09d
13 files changed