Adding a 5% as packet loss level for Opus

This is a follow up of
https://webrtc-codereview.appspot.com/16979004/

The purpose of this CL is to add 5% as a level for optimizing the packet loss rate to report to Opus. Adding such a level makes the grid finer.

BUG=
R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/13179004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6902 4adac7df-926f-26a2-2b94-8c16560cd09d