[Sync] Initial implementation of foreign sessions suggestions provider.

components/ntp_snippets/pref_util.h

Index: components/ntp_snippets/pref_util.h
diff --git a/components/ntp_snippets/pref_util.h b/components/ntp_snippets/pref_util.h
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+++ b/components/ntp_snippets/pref_util.h
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+// 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,
+                                                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_