Move prefs::kLastPolicyStatisticsUpdate to the policy component.

components/policy/core/common/policy_pref_names.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 "components/policy/core/common/policy_pref_names.h"
+
+namespace policy {
+namespace prefs {
+
+// 64-bit serialization of the time last policy usage statistics were collected
+// by UMA_HISTOGRAM_ENUMERATION.
+const char kLastPolicyStatisticsUpdate[] = "policy.last_statistics_update";
+
+}  // namespace prefs
+}  // namespace policy