[Sync] Initial implementation of foreign sessions suggestions provider.

components/ntp_snippets/pref_util.cc

+// 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.
+
+#include "components/ntp_snippets/pref_util.h"
+
+#include <memory>
+
+#include "base/values.h"
+#include "components/prefs/pref_service.h"
+
+namespace ntp_snippets {
+namespace prefs {
+
+std::set<std::string> ReadDismissedIDsFromPrefs(const PrefService& pref_service,
+                                                const std::string& pref_name) {
+  std::set<std::string> dismissed_ids;
+  const base::ListValue* list = pref_service.GetList(pref_name);
+  for (const std::unique_ptr<base::Value>& value : *list) {
+    std::string dismissed_id;
+    bool success = value->GetAsString(&dismissed_id);
+    DCHECK(success) << "Failed to parse dismissed id from prefs param "
+                    << pref_name << " into string.";
+    dismissed_ids.insert(dismissed_id);
+  }
+  return dismissed_ids;
+}
+
+void StoreDismissedIDsToPrefs(PrefService* pref_service,
+                              const std::string& pref_name,
+                              const std::set<std::string>& dismissed_ids) {
+  base::ListValue list;
+  for (const std::string& dismissed_id : dismissed_ids) {
+    list.AppendString(dismissed_id);
+  }
+  pref_service->Set(pref_name, list);
+}
+
+}  // namespace prefs
+}  // namespace ntp_snippets