Enable -Wsentinel on linux32, take 2.

chrome/browser/ui/libgtk2ui/gtk2_ui.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 "chrome/browser/ui/libgtk2ui/gtk2_ui.h"
 
 #include <set>
 
 #include <pango/pango.h>
 
@@ skipping 46 matching lines @@
 #endif
 
 // A minimized port of GtkThemeService into something that can provide colors
 // and images for aura.
 //
 // TODO(erg): There's still a lot that needs ported or done for the first time:
 //
 // - Render and inject the omnibox background.
 // - Make sure to test with a light on dark theme, too.
 
+// Work around a header bug:
+// linux/debian_wheezy_i386-sysroot/usr/include/linux/stddef.h redefines NULL
+// to 0, which breaks -Wsentinel. Get back the normal definition of NULL.
+// TODO(thakis): Remove this once we update sysroots.
+#define __need_NULL
+#include <stddef.h>
+
 namespace libgtk2ui {
 
 namespace {
 
 struct GObjectDeleter {
   void operator()(void* ptr) {
     g_object_unref(ptr);
   }
 };
 struct GtkIconInfoDeleter {
@@ skipping 1259 matching lines @@
   ClearAllThemeData();
   LoadGtkValues();
   NativeThemeGtk2::instance()->NotifyObservers();
 }
 
 }  // namespace libgtk2ui
 
 views::LinuxUI* BuildGtk2UI() {
   return new libgtk2ui::Gtk2UI;
 }