Remove contention between RTCP packets and encoding.

Receiving RTCP often caused the worker thread to stall for >20 ms
(>100ms observed) due to contention on VideoSender's send_crit_ (used to
protect encoding).

This change removes an unnecessary acquire of send_crit_ and caches
encoder settings in ViEEncoder instead of acquiring them through
::SendCodec() in VCM (which is blocking).

BUG=webrtc:5106
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1433703002 .

Cr-Commit-Position: refs/heads/master@{#10582}

webrtc/modules/video_coding/main/source/video_sender.cc

@@ -369,7 +369,6 @@ void VideoSender::SuspendBelowMinBitrate() {
 }
 
 bool VideoSender::VideoSuspended() const {
-  rtc::CritScope lock(&send_crit_);
   return _mediaOpt.IsVideoSuspended();
 }
 }  // namespace vcm