Late initialize MediaController, for less resource i.e. ProcessThread, usage by PeerConnection.

BUG=chromium:582441

Review URL: https://codereview.webrtc.org/1713043002

Cr-Commit-Position: refs/heads/master@{#11834}

webrtc/api/webrtcsession.cc

@@ -648,6 +648,7 @@ void WebRtcSession::Close() {
   ASSERT(!voice_channel_);
   ASSERT(!video_channel_);
   ASSERT(!data_channel_);
+  media_controller_->Close();
 }
 
 cricket::BaseChannel* WebRtcSession::GetChannel(