NetEq: Add thread annotation to const scoped_ptrs

Since the objects pointed to are not const, only the pointer to them,
they too must be accessed under lock.

Move the crit_sect to above the variables it is protecting.

R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/12679006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6340 4adac7df-926f-26a2-2b94-8c16560cd09d