Get the default device scale factor from monitor

ui/gfx/screen.h

 // 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.
 
 #ifndef UI_GFX_SCREEN_H_
 #define UI_GFX_SCREEN_H_
 #pragma once
 
+#include "ui/base/ui_export.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/gfx/monitor.h"
 #include "ui/gfx/point.h"
 
 namespace gfx {
 class Rect;
 class ScreenImpl;
 
 // A utility class for getting various info about screen size, monitors,
 // cursor position, etc.
@@ skipping 28 matching lines @@
   // Returns the bounds of the work area of the primary monitor.
   static gfx::Monitor GetPrimaryMonitor();
 
   // Returns the monitor that most closely intersects the provided bounds.
   static gfx::Monitor GetMonitorMatching(const gfx::Rect& match_rect);
 };
 
 }  // namespace gfx
 
 #endif  // VIEWS_SCREEN_H_