Fix Pango font rendering with HiDPi displays on Linux.

ui/gfx/linux_font_delegate.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 "ui/gfx/linux_font_delegate.h"
 
 namespace {
 
 gfx::LinuxFontDelegate* g_linux_font_delegate = 0;
 
 }  // namespace
 
 namespace gfx {
 
 void LinuxFontDelegate::SetInstance(LinuxFontDelegate* instance) {
   g_linux_font_delegate = instance;
 }
 
-const LinuxFontDelegate* LinuxFontDelegate::instance() {
+LinuxFontDelegate* LinuxFontDelegate::instance() {
   return g_linux_font_delegate;
 }
 
 }  // namespace gfx