Get the default device scale factor from monitor

ui/aura/monitor_manager.h

Index: ui/aura/monitor_manager.h
diff --git a/ui/aura/monitor_manager.h b/ui/aura/monitor_manager.h
index b3886cf3e5700bc29018fe6b3b5fabc057ffee3e..d6772437198ed9bfb7bef5c83c582af51a40bfe6 100644
--- a/ui/aura/monitor_manager.h
+++ b/ui/aura/monitor_manager.h
@@ -42,6 +42,8 @@ class AURA_EXPORT MonitorManager {
   // 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.