Basic UkmService implementation

components/ukm/ukm_pref_names.cc

+// Copyright 2017 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/ukm/ukm_pref_names.h"
+
+namespace ukm {
+namespace prefs {
+
+// A random uint64 value unique for each chrome install.
+const char kUkmClientId[] = "ukm.client_id";
+
+// Preference which stores serialized UKM logs to be uploaded.
+const char kUkmPersistedLogs[] = "ukm.persisted_logs";

[battre, 2017/01/12 08:12:55]

Do we really want to store this in prefs? Any update will trigger a rewrite of
prefs and prefs are not particularly fast to read and write.

Also, please make sure that clearing your browsing data (i.e. history) erases
all buffered UKMs.

[Steven Holte, 2017/01/13 23:15:26]

I'm pretty much just reusing the UMA logic here.  These logs are generated on a
similar interval to UMA (about every 5 minutes), and usually only get persisted
to disk on shutdown/background, so I wouldn't expect the pref write overhead to
be too large, but I'd be interested if there is a better suggestion for where to
persist buffered logs.

Added code to clear this data on Clear Browsing History.

+
+}  // namespace prefs
+}  // namespace ukm