Turned UiStrings into a singleton so that the continue window does not depend on ChromotingHost.

remoting/host/desktop_session_agent.cc

Index: remoting/host/desktop_session_agent.cc
diff --git a/remoting/host/desktop_session_agent.cc b/remoting/host/desktop_session_agent.cc
index a09e64395050939d3abc9381ea3e6b025f52e215..39a2aece8d81cde70c63421fb2407c5306e08e2e 100644
--- a/remoting/host/desktop_session_agent.cc
+++ b/remoting/host/desktop_session_agent.cc
@@ -195,9 +195,9 @@ void DesktopSessionAgent::OnStartSessionAgent(
       base::Bind(&DesktopSessionAgent::DisconnectSession, this);
 
   // Create the disconnect window.
-  disconnect_window_ = DisconnectWindow::Create();
+  disconnect_window_ = DisconnectWindow::Create(&ui_strings_);
   disconnect_window_->Show(
-      ui_strings_, disconnect_session,
+      disconnect_session,
       authenticated_jid.substr(0, authenticated_jid.find('/')));
 
   // Start monitoring local input.