[cast_channel] Make CastSocketService a global leaky singleton

components/cast_channel/cast_socket.h

Index: components/cast_channel/cast_socket.h
diff --git a/components/cast_channel/cast_socket.h b/components/cast_channel/cast_socket.h
index 566993b533a88f1125dfd67bdd4189190fa6a9b6..31d829eef3d0e0dae2adf236f4c86eaa71664306 100644
--- a/components/cast_channel/cast_socket.h
+++ b/components/cast_channel/cast_socket.h
@@ -129,6 +129,9 @@ class CastSocket {
 
   // Registers |observer| with the socket to receive messages and error events.
   virtual void AddObserver(Observer* observer) = 0;
+
+  // Unregisters |observer|.
+  virtual void RemoveObserver(Observer* observer) = 0;
 };
 
 // This class implements a channel between Chrome and a Cast device using a TCP
@@ -181,6 +184,7 @@ class CastSocketImpl : public CastSocket {
   bool keep_alive() const override;
   bool audio_only() const override;
   void AddObserver(Observer* observer) override;
+  void RemoveObserver(Observer* observer) override;
 
  protected:
   // CastTransport::Delegate methods for receiving handshake messages.