Fix crashers in GetPrimaryDisplay

ash/screen_ash.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/screen_ash.h"
 
 #include "ash/display/display_controller.h"
 #include "ash/display/multi_display_manager.h"
 #include "ash/shell.h"
 #include "ash/wm/coordinate_conversion.h"
@@ skipping 99 matching lines @@
 
 gfx::Display ScreenAsh::GetDisplayNearestPoint(const gfx::Point& point) const {
   return GetDisplayManager()->GetDisplayNearestPoint(point);
 }
 
 gfx::Display ScreenAsh::GetDisplayMatching(const gfx::Rect& match_rect) const {
   return GetDisplayManager()->GetDisplayMatching(match_rect);
 }
 
 gfx::Display ScreenAsh::GetPrimaryDisplay() const {
-  return Shell::GetInstance()->display_controller()->primary_display();
+  return DisplayController::GetPrimaryDisplay();
 }
 
 }  // namespace ash