Fix the captured screen rect conversion.
device_mode.dmPosition is already relative to the primary display's top-left, while the expected value of GetScreenRect() is also relative to the primary display's top-left.

TESTED=verified on Windows single monitor capturing and cursor capturing is fixed.

BUG=https://code.google.com/p/chromium/issues/detail?id=362631
R=wez@chromium.org

Review URL: https://webrtc-codereview.appspot.com/11789006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5890 4adac7df-926f-26a2-2b94-8c16560cd09d