Remember the configured device scale factor for external displays.

ash/display/display_change_observer_chromeos.cc

Index: ash/display/display_change_observer_chromeos.cc
diff --git a/ash/display/display_change_observer_chromeos.cc b/ash/display/display_change_observer_chromeos.cc
index 63cc33a7152ee641d363a0c6c59b5b04d57dc273..2634622b3de7537cd6db1a19a0f18035361ae51e 100644
--- a/ash/display/display_change_observer_chromeos.cc
+++ b/ash/display/display_change_observer_chromeos.cc
@@ -191,10 +191,16 @@ void DisplayChangeObserver::OnDisplayModeChanged(
       continue;
 
     float device_scale_factor = 1.0f;
-    if (!ui::IsDisplaySizeBlackListed(state.display->physical_size())) {
+    if (state.display->type() == ui::DISPLAY_CONNECTION_TYPE_INTERNAL) {
       device_scale_factor =
           FindDeviceScaleFactor((kInchInMm * mode_info->size().width() /
                                  state.display->physical_size().width()));
+    } else {
+      DisplayMode mode;
+      if (Shell::GetInstance()->display_manager()->GetSelectedModeForDisplayId(
+              state.display->display_id(), &mode)) {
+        device_scale_factor = mode.device_scale_factor;
+      }
     }
     gfx::Rect display_bounds(state.display->origin(), mode_info->size());