ipc: Remove unnecessary attachment broker plumbing.

remoting/host/desktop_session_proxy.cc

Index: remoting/host/desktop_session_proxy.cc
diff --git a/remoting/host/desktop_session_proxy.cc b/remoting/host/desktop_session_proxy.cc
index 54399d98285b70656dc1fa0cd87973acd4d24c87..ba098f69e6224306d0b7858fad844760f1ef6dc7 100644
--- a/remoting/host/desktop_session_proxy.cc
+++ b/remoting/host/desktop_session_proxy.cc
@@ -265,10 +265,8 @@ bool DesktopSessionProxy::AttachToDesktop(
 
   // Connect to the desktop process.
   desktop_channel_ = IPC::ChannelProxy::Create(desktop_channel_handle,
-                                               IPC::Channel::MODE_CLIENT,
-                                               this,
-                                               io_task_runner_.get(),
-                                               nullptr);
+                                               IPC::Channel::MODE_CLIENT, this,
+                                               io_task_runner_.get());
 
   // Pass ID of the client (which is authenticated at this point) to the desktop
   // session agent and start the agent.