Add/update checks to diagnose the NPE issue

ash/display/display_controller.cc

 // Copyright (c) 2012 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_controller.h"
 
 #include <algorithm>
 #include <cmath>
 #include <map>
 
@@ skipping 612 matching lines @@
   // When the primary root window's display is removed, move the primary
   // root to the other display.
   if (primary_display_id == display.id()) {
     // Temporarily store the primary root window in
     // |primary_root_window_for_replace_| when replacing the display.
     if (window_tree_hosts_.size() == 0) {
       primary_display_id = gfx::Display::kInvalidDisplayID;
       primary_tree_host_for_replace_ = host_to_delete;
       return;
     }
-    DCHECK_EQ(1U, window_tree_hosts_.size());
     primary_display_id = ScreenUtil::GetSecondaryDisplay().id();
+    CHECK_NE(gfx::Display::kInvalidDisplayID, primary_display_id);
+
+    CHECK_EQ(1U, window_tree_hosts_.count(primary_display_id));
+
     AshWindowTreeHost* primary_host = host_to_delete;
-
     // Delete the other host instead.
     host_to_delete = window_tree_hosts_[primary_display_id];
     GetRootWindowSettings(GetWindow(host_to_delete))->display_id = display.id();
 
     // Setup primary root.
     window_tree_hosts_[primary_display_id] = primary_host;
     GetRootWindowSettings(GetWindow(primary_host))->display_id =
         primary_display_id;
 
     OnDisplayMetricsChanged(
@@ skipping 159 matching lines @@
     std::string name =
         root_windows[i] == primary ? "aura_root_0" : "aura_root_x";
     gfx::AcceleratedWidget xwindow =
         root_windows[i]->GetHost()->GetAcceleratedWidget();
     XStoreName(gfx::GetXDisplay(), xwindow, name.c_str());
   }
 #endif
 }
 
 }  // namespace ash