Show message to indicate that mirriring with more than 3 display is not supported.

ash/display/display_configuration_controller.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/display/display_configuration_controller.h"
 
 #include "ash/display/display_animator.h"
 #include "ash/display/display_manager.h"
+#include "ash/display/display_util.h"
 #include "ash/rotator/screen_rotation_animator.h"
 #include "ash/screen_util.h"
 #include "base/time/time.h"
 #include "ui/display/manager/display_layout.h"
 
 #if defined(OS_CHROMEOS)
 #include "ash/display/display_animator_chromeos.h"
 #include "base/sys_info.h"
+#include "grit/ash_strings.h"
 #endif
 
 namespace {
 
 // Specifies how long the display change should have been disabled
 // after each display change operations.
 // |kCycleDisplayThrottleTimeoutMs| is set to be longer to avoid
 // changing the settings while the system is still configurating
 // displays. It will be overriden by |kAfterDisplayChangeThrottleTimeoutMs|
 // when the display change happens, so the actual timeout is much shorter.
@@ skipping 47 matching lines @@
     display_animator_->StartFadeOutAnimation(
         base::Bind(&DisplayConfigurationController::SetDisplayLayoutImpl,
                    weak_ptr_factory_.GetWeakPtr(), base::Passed(&layout)));
   } else {
     SetDisplayLayoutImpl(std::move(layout));
   }
 }
 
 void DisplayConfigurationController::SetMirrorMode(bool mirror,
                                                    bool user_action) {
+  if (display_manager_->num_connected_displays() > 2) {
+#if defined(OS_CHROMEOS)
+    if (user_action)
+      ShowDisplayErrorNotification(IDS_ASH_DISPLAY_MIRRORING_NOT_SUPPORTED);
+#endif
+    return;
+  }
   if (display_manager_->num_connected_displays() <= 1 ||
       display_manager_->IsInMirrorMode() == mirror || IsLimited()) {
     return;
   }
   SetThrottleTimeout(kCycleDisplayThrottleTimeoutMs);
   if (user_action && display_animator_) {
     display_animator_->StartFadeOutAnimation(
         base::Bind(&DisplayConfigurationController::SetMirrorModeImpl,
                    weak_ptr_factory_.GetWeakPtr(), mirror));
   } else {
@@ skipping 67 matching lines @@
 }
 
 void DisplayConfigurationController::SetPrimaryDisplayIdImpl(
     int64_t display_id) {
   window_tree_host_manager_->SetPrimaryDisplayId(display_id);
   if (display_animator_)
     display_animator_->StartFadeInAnimation();
 }
 
 }  // namespace ash