Remove mouse cursor capturer from the ScreenCapturer interface

Mouse can be captured using MouseCursorMonitor and all code in chromium
already uses it instead of ScreenCapturer.

R=jiayl@webrtc.org

Committed: https://code.google.com/p/webrtc/source/detail?r=7363

Review URL: https://webrtc-codereview.appspot.com/31529004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7365 4adac7df-926f-26a2-2b94-8c16560cd09d