Get the default device scale factor from monitor

ui/aura/monitor_manager.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_AURA_MONITOR_MANAGER_H_
 #define UI_AURA_MONITOR_MANAGER_H_
 #pragma once
 
 #include <string>
 #include <vector>
@@ skipping 24 matching lines @@
   }
   static bool use_fullscreen_host_window() {
     return use_fullscreen_host_window_;
   }
 
   // Creates a monitor from string spec. 100+200-1440x800 creates monitor
   // whose size is 1440x800 at the location (100, 200) in screen's coordinates.
   // The location can be omitted and be just "1440x800", which creates
   // monitor at the origin of the screen. An empty string creates
   // the monitor with default size.
+  //  The device scale factor can be specified by "*", like "1280x780*2",
+  // or |gfx::Monitor::GetDefaultDeviceScaleFactor()| will be used if omitted.
   static gfx::Monitor CreateMonitorFromSpec(const std::string& spec);
 
   // A utility function to create a root window for primary monitor.
   static RootWindow* CreateRootWindowForPrimaryMonitor();
 
   MonitorManager();
   virtual ~MonitorManager();
 
   // Adds/removes MonitorObservers.
   void AddObserver(MonitorObserver* observer);
@@ skipping 35 matching lines @@
   // switches::kAuraHostWindowSize flag.
   static bool use_fullscreen_host_window_;
 
   ObserverList<MonitorObserver> observers_;
   DISALLOW_COPY_AND_ASSIGN(MonitorManager);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MONITOR_MANAGER_H_