ash: Do a null-check for the screen-position client.

ash/wm/coordinate_conversion.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/wm/coordinate_conversion.h"
 
 #include "ash/display/display_controller.h"
 #include "ash/shell.h"
 #include "ui/aura/client/screen_position_client.h"
 #include "ui/gfx/display.h"
@@ skipping 14 matching lines @@
       GetRootWindowForDisplayId(display.id());
 }
 
 aura::Window* GetRootWindowMatching(const gfx::Rect& rect) {
   const gfx::Display& display = Shell::GetScreen()->GetDisplayMatching(rect);
   return Shell::GetInstance()->display_controller()->
       GetRootWindowForDisplayId(display.id());
 }
 
 void ConvertPointToScreen(const aura::Window* window, gfx::Point* point) {
-  CHECK(window);
-  CHECK(window->GetRootWindow());
-  CHECK(aura::client::GetScreenPositionClient(window->GetRootWindow()));
+  // It is possible for the root window to not have a screen position client
+  // when switching multi-monitor mode from extended to mirror.
+  if (!aura::client::GetScreenPositionClient(window->GetRootWindow()))
+    return;
   aura::client::GetScreenPositionClient(window->GetRootWindow())->
       ConvertPointToScreen(window, point);
 }
 
 void ConvertPointFromScreen(const aura::Window* window,
                             gfx::Point* point_in_screen) {
   aura::client::GetScreenPositionClient(window->GetRootWindow())->
       ConvertPointFromScreen(window, point_in_screen);
 }
 
 }  // namespace wm
 }  // namespace ash