Use ScreenCapturer to capture the whole and clip to the window rect when the shared window is on the top.

BUG=crbug/403703, crbug/316603
R=wez@chromium.org

Review URL: https://webrtc-codereview.appspot.com/22759004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7684 4adac7df-926f-26a2-2b94-8c16560cd09d