- Pad packets (empty) were often NACKed even though they were received.
- Padding only frames (empty) didn't properly update the decoding state,
and would therefore be NACKed even though they were received.
TEST=trybots
BUG=1150
Review URL: https://webrtc-codereview.appspot.com/966026
git-svn-id: http://webrtc.googlecode.com/svn/trunk@3181 4adac7df-926f-26a2-2b94-8c16560cd09d