DTX disable mode will be removed as a part of the ACM redesign work. This CL effectively reverts r8129, and relands r8125, but now using assert instead of DCHECK. COAUTHOR:kwiberg@webrtc.org TBR=henrika@webrtc.org Review URL: https://webrtc-codereview.appspot.com/37839004 Cr-Commit-Position: refs/heads/master@{#8185} git-svn-id: http://webrtc.googlecode.com/svn/trunk@8185 4adac7df-926f-26a2-2b94-8c16560cd09d