Reland of Enable RL by default

components/reading_list/core/reading_list.gni

Index: components/reading_list/core/reading_list.gni
diff --git a/components/reading_list/core/reading_list.gni b/components/reading_list/core/reading_list.gni
new file mode 100644
index 0000000000000000000000000000000000000000..26c4e1e7cb59563b3d163fc8ab4ea8c0c47cb4f8
--- /dev/null
+++ b/components/reading_list/core/reading_list.gni
@@ -0,0 +1,9 @@
+# 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.
+
+declare_args() {
+  # Controls whether reading list support is active or not. Currently only
+  # supported on iOS (on other platforms, the feature is always disabled).
+  enable_reading_list = true
+}