Do not call CaptureCursor in ScreenCapturerWinGdi if no MouseShapeObserver.
It's wasted work and affects frame rate adaptation in Chrome.

BUG=
R=sergeyu@chromium.org

Review URL: https://webrtc-codereview.appspot.com/19789004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6499 4adac7df-926f-26a2-2b94-8c16560cd09d