Convert webrtc/video/ abort/assert to CHECK/DCHECK.

Also replaces NULL with nullptr. This gives nicer error messages and
keeps style consistent.

BUG=1756
R=magjed@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/42879004

Cr-Commit-Position: refs/heads/master@{#8831}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8831 4adac7df-926f-26a2-2b94-8c16560cd09d
13 files changed