[Sync] Initial implementation of foreign sessions suggestions provider.

components/ntp_snippets/pref_util.h

+// Copyright 2016 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.
+
+#ifndef COMPONENTS_NTP_SNIPPETS_PREF_UTIL_H_
+#define COMPONENTS_NTP_SNIPPETS_PREF_UTIL_H_
+
+#include <set>
+#include <string>
+
+class PrefService;
+
+namespace ntp_snippets {
+namespace prefs {
+
+// Reads a given preference and then deserializes it into a set of strings.
+std::set<std::string> ReadDismissedIDsFromPrefs(const PrefService& pref_service,

[battre, 2016/09/19 08:54:34]

nit: I think that in the majority of cases, we pass this as a "const
PrefService* pref_service" today. I don't know whether any approach is better
than the other. (just a thing of consistency)

[tschumann, 2016/09/19 12:19:23]

If nullptr is not allowed and a method does not keep a reference to an input
parameter, const reference is preferred over const pointer.
(https://engdoc.corp.google.com/eng/doc/devguide/cpp/styleguide.shtml?cl=head#...)
life-time requirements are very clear then at the call site.

[skym, 2016/09/19 18:54:47]

Acknowledged.

+                                                const std::string& pref_name);
+
+// Serializes a set of strings into a given preference.
+void StoreDismissedIDsToPrefs(PrefService* pref_service,
+                              const std::string& pref_name,
+                              const std::set<std::string>& dismissed_ids);
+
+}  // namespace prefs
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_PREF_UTIL_H_