It appears there the encode and send operation can happen over multiple
threads. Also, padding data itself may be sent on a different thread.
Remove thread checker and protect all data with crit_sect.
BUG=webrtc:5998
TBR=stefan@webrtc.org
Review-Url: https://codereview.webrtc.org/2066863002
Cr-Commit-Position: refs/heads/master@{#13137}