Drop early packets when not sending in TransportAdapter.

Particularly, suppress periodic RTCP packets before
VideoSendStream.StartSending() or VideoReceiveStream.StartReceiving() have been called, respectively.

RTCP packets are sent periodically, by the Process thread, for every ViE channel even those not sending.

BUG=
R=pbos@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/7569004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5438 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/video/video_send_stream.cc b/video/video_send_stream.cc
index a9fd50d..3b592f8 100644
--- a/video/video_send_stream.cc
+++ b/video/video_send_stream.cc
@@ -259,6 +259,7 @@
 VideoSendStreamInput* VideoSendStream::Input() { return this; }
 
 void VideoSendStream::StartSending() {
+  transport_adapter_.Enable();
   video_engine_base_->StartSend(channel_);
   video_engine_base_->StartReceive(channel_);
 }
@@ -266,6 +267,7 @@
 void VideoSendStream::StopSending() {
   video_engine_base_->StopSend(channel_);
   video_engine_base_->StopReceive(channel_);
+  transport_adapter_.Disable();
 }
 
 bool VideoSendStream::SetCodec(const VideoCodec& codec) {