Adding an Android about:flags feature for offline previews

components/previews/core/previews_features.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/previews/core/previews_features.h"
+
+namespace previews {
+namespace features {
+
+// Enables the Offline previews on Android.
+const base::Feature kOfflinePreviews{"OfflinePreviews",
+                                     base::FEATURE_DISABLED_BY_DEFAULT};
+
+}  // namespace features
+}  // namespace previews