[cast_channel] Implement CastSocketService::OpenSocket()

components/cast_channel/cast_channel_enum.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_CAST_CHANNEL_CAST_CHANNEL_ENUM_H_
 #define COMPONENTS_CAST_CHANNEL_CAST_CHANNEL_ENUM_H_
 
 #include <string>
 
 namespace cast_channel {
 
 // Helper function to convert scoped enums to their underlying type, for use
 // with ostreams.
 template <typename Enumeration>
 auto AsInteger(Enumeration const value) ->
     typename std::underlying_type<Enumeration>::type {
   return static_cast<typename std::underlying_type<Enumeration>::type>(value);
 }
 
 // Maps to enum ReadyState in cast_channel.idl
 enum class ReadyState {
   NONE,
   CONNECTING,
   OPEN,
-  CLOSING,
+  CLOSING,  // TODO(zhaobin): Remove this value because it is unused.
   CLOSED,
 };
 
 // Maps to enum ChannelError in cast_channel.idl
 enum class ChannelError {
   NONE,
   CHANNEL_NOT_OPEN,
   AUTHENTICATION_ERROR,
   CONNECT_ERROR,
   CAST_SOCKET_ERROR,
@@ skipping 99 matching lines @@
   WRITE_ERROR,  // Terminal states here and below.
   IDLE,
 };
 
 std::string ReadyStateToString(ReadyState ready_state);
 std::string ChannelErrorToString(ChannelError channel_error);
 
 }  // namespace cast_channel
 
 #endif  // COMPONENTS_CAST_CHANNEL_CAST_CHANNEL_ENUM_H_