Get the default device scale factor from monitor

ui/aura/single_monitor_manager.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 "ui/aura/single_monitor_manager.h"
 
 #include <string>
 
 #include "base/command_line.h"
 #include "ui/aura/aura_switches.h"
@@ skipping 70 matching lines @@
 }
 
 void SingleMonitorManager::OnWindowDestroying(Window* window) {
   if (root_window_ == window)
     root_window_ = NULL;
 }
 
 void SingleMonitorManager::Init() {
   const string size_str = CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
       switches::kAuraHostWindowSize);
-  monitor_ = CreateMonitorFromSpec(size_str);
+  monitor_ = CreateMonitorFromSpec(default_device_scale_factor(), size_str);
 }
 
 void SingleMonitorManager::Update(const gfx::Size size) {
   monitor_.SetSize(size);
 }
 
 }  // namespace aura