Switch to standard integer types in base/prefs/.

BUG=138542
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Review URL: https://codereview.chromium.org/1544033003

Cr-Commit-Position: refs/heads/master@{#366805}
diff --git a/base/prefs/pref_notifier_impl_unittest.cc b/base/prefs/pref_notifier_impl_unittest.cc
index c3cbf4f..cb3c3b5 100644
--- a/base/prefs/pref_notifier_impl_unittest.cc
+++ b/base/prefs/pref_notifier_impl_unittest.cc
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <stddef.h>
+
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/prefs/mock_pref_change_callback.h"