Hookup device dpi for ChromeOS

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 fd2a66d1fa86e2aeb367725a758722e11a40b590..4ca4df453d6d4d595ff8036a8a5b35d8ed895096 100644
--- a/ash/display/display_change_observer_chromeos.cc
+++ b/ash/display/display_change_observer_chromeos.cc
@@ -187,12 +187,11 @@ void DisplayChangeObserver::OnDisplayModeChanged(
       continue;
 
     float device_scale_factor = 1.0f;
+    float dpi = kInchInMm * mode_info->size().width() /
+        state->physical_size().width();

[oshima, 2015/08/19 02:07:08]

This is physical DPI, and it may not represent the UI size that user is using
when DSF and/or UI zooming are applied. Will this really work for your scenario?

[bshe, 2015/08/19 13:37:09]

right. So I called this one device_dpi instead of dpi. And then in
display_info_provider_chromeos.cc. I divide the device_dpi by
device_scale_factor to calculate the current effective dpi. It looks like
working. On pixel, the size that I calcuated from dpi seems correct(tried
1280x850 and 2560x1700).

[oshima, 2015/08/20 00:20:32]

What about 1440x956?

[bshe, 2015/08/20 14:42:05]

I see. 1440x956 will return the wrong dpi as scale factor is tied to device dpi
threshold. I have changed the code so the dpi is calculated from device_dpi,
native screen size and current effective screen size.    

     if (state->type() == ui::DISPLAY_CONNECTION_TYPE_INTERNAL) {
-      if (!ui::IsDisplaySizeBlackListed(state->physical_size())) {
-        device_scale_factor =
-            FindDeviceScaleFactor((kInchInMm * mode_info->size().width() /
-                                   state->physical_size().width()));
-      }
+      if (!ui::IsDisplaySizeBlackListed(state->physical_size()))
+        device_scale_factor = FindDeviceScaleFactor(dpi);
     } else {
       DisplayMode mode;
       if (Shell::GetInstance()->display_manager()->GetSelectedModeForDisplayId(
@@ -234,6 +233,7 @@ void DisplayChangeObserver::OnDisplayModeChanged(
     new_info.set_native(true);
     new_info.set_is_aspect_preserving_scaling(
         state->is_aspect_preserving_scaling());
+    new_info.set_device_dpi(dpi);
 
     std::vector<DisplayMode> display_modes =
         (state->type() == ui::DISPLAY_CONNECTION_TYPE_INTERNAL)