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}
diff --git a/webrtc/api/webrtcsession.cc b/webrtc/api/webrtcsession.cc
index 6bfa8a5..455be0f 100644
--- a/webrtc/api/webrtcsession.cc
+++ b/webrtc/api/webrtcsession.cc
@@ -648,6 +648,7 @@
   ASSERT(!voice_channel_);
   ASSERT(!video_channel_);
   ASSERT(!data_channel_);
+  media_controller_->Close();
 }
 
 cricket::BaseChannel* WebRtcSession::GetChannel(