Remember the configured device scale factor for external displays.

ash/display/display_change_observer_chromeos.cc

 // Copyright 2013 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.
 
 #include "ash/display/display_change_observer_chromeos.h"
 
 #include <algorithm>
 #include <map>
 #include <set>
 #include <vector>
@@ skipping 173 matching lines @@
     if (state.display->type() == ui::DISPLAY_CONNECTION_TYPE_INTERNAL &&
         gfx::Display::InternalDisplayId() == gfx::Display::kInvalidDisplayID) {
       gfx::Display::SetInternalDisplayId(state.display->display_id());
     }
 
     const ui::DisplayMode* mode_info = state.display->current_mode();
     if (!mode_info)
       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());
 
     std::string name =
         state.display->type() == ui::DISPLAY_CONNECTION_TYPE_INTERNAL ?
             l10n_util::GetStringUTF8(IDS_ASH_INTERNAL_DISPLAY_NAME) :
             state.display->display_name();
     if (name.empty())
       name = l10n_util::GetStringUTF8(IDS_ASH_STATUS_TRAY_UNKNOWN_DISPLAY_NAME);
 
@@ skipping 40 matching lines @@
 // static
 float DisplayChangeObserver::FindDeviceScaleFactor(float dpi) {
   for (size_t i = 0; i < arraysize(kThresholdTable); ++i) {
     if (dpi > kThresholdTable[i].dpi)
       return kThresholdTable[i].device_scale_factor;
   }
   return 1.0f;
 }
 
 }  // namespace ash