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..1df63bde4077daa23ac49c71c0b65a5a4fb49cd4 100644
--- a/ash/display/display_change_observer_chromeos.cc
+++ b/ash/display/display_change_observer_chromeos.cc
@@ -191,10 +191,18 @@ void DisplayChangeObserver::OnDisplayModeChanged(
       continue;
 
     float device_scale_factor = 1.0f;
-    if (!ui::IsDisplaySizeBlackListed(state.display->physical_size())) {
-      device_scale_factor =
-          FindDeviceScaleFactor((kInchInMm * mode_info->size().width() /
-                                 state.display->physical_size().width()));
+    if (state.display->type() == ui::DISPLAY_CONNECTION_TYPE_INTERNAL) {
+      if (!ui::IsDisplaySizeBlackListed(state.display->physical_size())) {

[oshima, 2014/08/07 23:03:49]

internal display should not be black listed, so we can remove this.

[Jun Mukai, 2014/08/08 00:57:33]

Done.

+        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());