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/mediacontroller.h b/webrtc/api/mediacontroller.h
index 7b6a2a3..d7c76ab 100644
--- a/webrtc/api/mediacontroller.h
+++ b/webrtc/api/mediacontroller.h
@@ -32,6 +32,7 @@
       cricket::ChannelManager* channel_manager);
 
   virtual ~MediaControllerInterface() {}
+  virtual void Close() = 0;
   virtual webrtc::Call* call_w() = 0;
   virtual cricket::ChannelManager* channel_manager() const = 0;
   virtual const cricket::MediaConfig& config() const = 0;