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

ui/views/linux_ui/linux_ui.h

 // 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.
 
 #ifndef UI_VIEWS_LINUX_UI_LINUX_UI_H_
 #define UI_VIEWS_LINUX_UI_LINUX_UI_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 136 matching lines @@
 
   // Notifies the window manager that start up has completed.
   // Normally Chromium opens a new window on startup and GTK does this
   // automatically. In case Chromium does not open a new window on startup,
   // e.g. an existing browser window already exists, this should be called.
   virtual void NotifyWindowManagerStartupComplete() = 0;
 
   // 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
 
 #endif  // UI_VIEWS_LINUX_UI_LINUX_UI_H_