[Metrics] Move CachingPermutedEntropyProvider into the variations component.

components/variations/pref_names.cc

+// Copyright 2014 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/variations/pref_names.h"
+
+namespace metrics {
+namespace prefs {
+
+// A serialized PermutedEntropyCache protobuf, used as a cache to avoid
+// recomputing permutations.
+const char kVariationsPermutedEntropyCache[] =
+    "user_experience_metrics.permuted_entropy_cache";
+
+}  // namespace prefs
+}  // namespace metrics