Fixed segmentation fault. Because currently return null when call views::LinuxUI::instance().

ui/views/widget/desktop_aura/desktop_screen_x11.cc

 // Copyright (c) 2012 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 "ui/views/widget/desktop_aura/desktop_screen_x11.h"
 
 #include <X11/extensions/Xrandr.h>
 #include <X11/Xlib.h>
 
 // It clashes with out RootWindow.
@@ skipping 269 matching lines @@
   gfx::Rect work_area_in_pixels;
   std::vector<int> value;
   if (ui::GetIntArrayProperty(x_root_window_, "_NET_WORKAREA", &value) &&
       value.size() >= 4) {
     work_area_in_pixels = gfx::Rect(value[0], value[1], value[2], value[3]);
     has_work_area = true;
   }
 
   // As per-display scale factor is not supported right now,
   // the X11 root window's scale factor is always used.
-  const float device_scale_factor =
-      views::LinuxUI::instance()->GetDeviceScaleFactor();
+  const float device_scale_factor = GetDeviceScaleFactor();
   for (int i = 0; i < resources->noutput; ++i) {
     RROutput output_id = resources->outputs[i];
     gfx::XScopedPtr<XRROutputInfo,
                     gfx::XObjectDeleter<XRROutputInfo, void, XRRFreeOutputInfo>>
         output_info(XRRGetOutputInfo(xdisplay_, resources.get(), output_id));
 
     bool is_connected = (output_info->connection == RR_Connected);
     if (!is_connected)
       continue;
 
@@ skipping 62 matching lines @@
   change_notifier_.NotifyDisplaysChanged(old_displays, displays_);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 
 gfx::Screen* CreateDesktopScreen() {
   return new DesktopScreenX11;
 }
 
 }  // namespace views