[Cast Channel] Add support for nonce challenge to Cast channel authentication.

extensions/browser/api/cast_channel/cast_socket.h

Index: extensions/browser/api/cast_channel/cast_socket.h
diff --git a/extensions/browser/api/cast_channel/cast_socket.h b/extensions/browser/api/cast_channel/cast_socket.h
index 2fa1821d7ca054fa369b02f37c06ae6c7036707f..9590e6853af2dca1f3ceca6d12949d4ede7316be 100644
--- a/extensions/browser/api/cast_channel/cast_socket.h
+++ b/extensions/browser/api/cast_channel/cast_socket.h
@@ -218,6 +218,8 @@ class CastSocketImpl : public CastSocket {
   friend class CastSocketTest;
   friend class TestCastSocket;
 
+  static void EnsureNonceTimely();
+
   void SetErrorState(ChannelError error_state) override;
 
   // Frees resources and cancels pending callbacks.  |ready_state_| will be set
@@ -283,6 +285,11 @@ class CastSocketImpl : public CastSocket {
   void SetConnectState(proto::ConnectionState connect_state);
   void SetReadyState(ReadyState ready_state);
 
+  // The nonce challenge to send to the Cast receiver.
+  // The nonce is updated daily.
+  static std::string nonce_;
+  static base::Time nonce_generation_time_;
+
   base::ThreadChecker thread_checker_;
 
   const std::string owner_extension_id_;