[Feed] Add empty pref_names files.

Bug: 831648
Change-Id: If86282e15d436bfa1e625f38941d59e47e3e5294
Reviewed-on: https://chromium-review.googlesource.com/1089887
Commit-Queue: Sky Malice <skym@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565298}

components/feed/core/BUILD.gn

@@ -20,6 +20,8 @@ source_set("feed_core") {
     "feed_scheduler_host.h",
     "feed_storage_database.cc",
     "feed_storage_database.h",
+    "pref_names.cc",
+    "pref_names.h",
     "time_serialization.cc",
     "time_serialization.h",
   ]

components/feed/core/pref_names.cc

+// Copyright 2018 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/feed/core/pref_names.h"
+
+namespace feed {
+
+namespace prefs {}  // namespace prefs
+
+}  // namespace feed

components/feed/core/pref_names.h

+// Copyright 2018 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_FEED_CORE_PREF_NAMES_H_
+#define COMPONENTS_FEED_CORE_PREF_NAMES_H_
+
+namespace feed {
+
+namespace prefs {}  // namespace prefs
+
+}  // namespace feed
+
+#endif  // COMPONENTS_FEED_CORE_PREF_NAMES_H_