Implement a work around for Chrome full-screen tab switch on Mac.

Chrome creates a new window in full-screen and minimizes the old window when a tab is switched to full-screen.
We try to find the new window to continue capturing for window sharing.

BUG=crbug/385294
R=sergeyu@chromium.org

Review URL: https://webrtc-codereview.appspot.com/19839004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6629 4adac7df-926f-26a2-2b94-8c16560cd09d