Adds DisplayErrorDialog to show the error message of the failure of mirroring to the users.

chromeos/display/output_configurator.cc

Index: chromeos/display/output_configurator.cc
diff --git a/chromeos/display/output_configurator.cc b/chromeos/display/output_configurator.cc
index 710575eb9ab0f0c6f2d6a866fc7e4865fbce0d44..0754b2711b42ad7e7c9422993138454576580e8d 100644
--- a/chromeos/display/output_configurator.cc
+++ b/chromeos/display/output_configurator.cc
@@ -740,9 +740,6 @@ bool OutputConfigurator::SetDisplayMode(OutputState new_state) {
       output_state_ == STATE_SINGLE)
     return false;
 
-  if (output_state_ == new_state)
-    return true;
-
   Display* display = base::MessagePumpAuraX11::GetDefaultXDisplay();
   CHECK(display != NULL);
   XGrabServer(display);