[Chromoting] Fix DCHECK when shutting down It2Me host from client side

remoting/host/it2me/it2me_host.cc

Index: remoting/host/it2me/it2me_host.cc
diff --git a/remoting/host/it2me/it2me_host.cc b/remoting/host/it2me/it2me_host.cc
index 0afaee6fc16d39d08523dec3c0784d66b83648a3..ac19c18a09d1f618c558dbf53ed3adc94c539a3e 100644
--- a/remoting/host/it2me/it2me_host.cc
+++ b/remoting/host/it2me/it2me_host.cc
@@ -102,6 +102,12 @@ void It2MeHost::Disconnect() {
 void It2MeHost::Shutdown() {
   DCHECK(host_context_->network_task_runner()->BelongsToCurrentThread());
 
+  // Shutdown is expected to be called twice, if OnClientDisconnected has been
+  // called. See http://crbug.com/617474 for details.

[Sergey Ulanov, 2016/06/08 21:54:04]

The link to the bug is not useful here. I would just remove the comment - the
code is self-explanatory.

[Hzj_jie, 2016/06/08 23:12:20]

I would prefer to keep it, since the condition of returning is a little bit
magic to me. i.e. One may need to search for the references of Shutdown function
to understand why the Shutdown function may be called twice in some scenarios.

[Sergey Ulanov, 2016/06/08 23:26:15]

Ok, then I suggest rewording:
 // Disconnect() may be called even when after the host been
 // already stopped. Ignore repeated calls.


Also it may be useful to rename Shutdown() to DisconnectOnNetworkThread().

+  if (state_ == kDisconnected) {
+    return;
+  }
+
   confirmation_dialog_proxy_.reset();
 
   host_event_logger_.reset();