Fix window capturing on Windows when the window is minimized.

BUG=crbug/410290
R=sergeyu@chromium.org

Review URL: https://webrtc-codereview.appspot.com/20319004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7158 4adac7df-926f-26a2-2b94-8c16560cd09d