device scale factor detection: use gtk-xft-dpi consistently

ui/views/linux_ui/linux_ui.h

Index: ui/views/linux_ui/linux_ui.h
diff --git a/ui/views/linux_ui/linux_ui.h b/ui/views/linux_ui/linux_ui.h
index 088de922431d345fe354ba927850cb942e5bed42..fa910d23f77a01da98fe20300b41aac73def3225 100644
--- a/ui/views/linux_ui/linux_ui.h
+++ b/ui/views/linux_ui/linux_ui.h
@@ -154,6 +154,9 @@ class VIEWS_EXPORT LinuxUI : public ui::LinuxInputMethodContextFactory,
   // Updates the device scale factor so that the default font size can be
   // recalculated.
   virtual void UpdateDeviceScaleFactor(float device_scale_factor) = 0;
+
+  // Determines the device scale factor of the primary screen.
+  virtual float GetDeviceScaleFactor() const = 0;
 };
 
 }  // namespace views