[remoting][FTL] Add back XMPP-based host listener for double signaling host

We can't just remove HostChangeNotificationListener from double
signaling hosts since the legacy directory server only notifies host
deletion through XMPP, and old clients will only talk to the legacy
server.

This CL ensures XMPP-based HostChangeNotificationListener is created
for both double signaling hosts.

Bug: 968828
Change-Id: I234a30cb5162f27a1db9510962f360ff54ae5685
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1638138
Auto-Submit: Yuwei Huang <yuweih@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665019}

remoting/host/remoting_me2me_host.cc

@@ -1526,10 +1526,14 @@ void HostProcess::InitializeSignaling() {
     signal_strategy_ = std::move(xmpp_signal_strategy);
     log_to_server_ = std::make_unique<XmppLogToServer>(
         ServerLogEntry::ME2ME, signal_strategy_.get(), directory_bot_jid_);
-    host_change_notification_listener_.reset(new HostChangeNotificationListener(
-        this, host_id_, unowned_xmpp_signal_strategy, directory_bot_jid_));
   }
 
+  // XMPP based HostChangeNotificationListener is still needed by double
+  // signaling hosts so that they can handle host deletion triggered by an old
+  // client talking to the legacy directory backend.
+  host_change_notification_listener_.reset(new HostChangeNotificationListener(
+      this, host_id_, unowned_xmpp_signal_strategy, directory_bot_jid_));
+
 #if defined(USE_GCD)
   // Create objects to manage GCD state.
   ServiceUrls* service_urls = ServiceUrls::GetInstance();