Get the default device scale factor from monitor

ui/aura/monitor_change_observer_x11.cc

Index: ui/aura/monitor_change_observer_x11.cc
diff --git a/ui/aura/monitor_change_observer_x11.cc b/ui/aura/monitor_change_observer_x11.cc
index 3239022a357424ea62c8cf6c0d0823f314d7d8a9..881dacd613c25754bc3ef58ff3e53f40224f8acf 100644
--- a/ui/aura/monitor_change_observer_x11.cc
+++ b/ui/aura/monitor_change_observer_x11.cc
@@ -64,6 +64,8 @@ bool MonitorChangeObserverX11::Dispatch(const base::NativeEvent& event) {
 void MonitorChangeObserverX11::NotifyMonitorChange() {
   if (!MonitorManager::use_fullscreen_host_window())
     return;  // Use the default monitor that monitor manager determined.
+  float default_device_scale_factor =
+      Env::GetInstance()->monitor_manager()->default_device_scale_factor();
 
   XRRScreenResources* screen_resources =
       XRRGetScreenResources(xdisplay_, x_root_window_);
@@ -101,6 +103,7 @@ void MonitorChangeObserverX11::NotifyMonitorChange() {
     monitors.push_back(gfx::Monitor(
         0,
         gfx::Rect(crtc_info->x, crtc_info->y, mode->width, mode->height)));
+    monitors.back().set_device_scale_factor(default_device_scale_factor);
     y_coords.insert(crtc_info->y);
     XRRFreeOutputInfo(output_info);
   }