Fix bugs in DesktopRegion::Subtract().

Fixed a bug that caused the crash in the linked bug and also couple of
other issues in the same code. Also added more tests.

BUG=crbug.com/295057
R=wez@chromium.org

Review URL: https://webrtc-codereview.appspot.com/2259005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4806 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed