Fix a cursor capturing issue on Windows.
The input position to WindowFromPoint should be relative to the desktop, not
relative to the window; if the result from WindowFromPoint is a child window
of the shared top window, it should be captured.

BUG=
R=sergeyu@chromium.org

Review URL: https://webrtc-codereview.appspot.com/7959004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5487 4adac7df-926f-26a2-2b94-8c16560cd09d