Reland of Enable RL by default

components/reading_list/core/reading_list.gni

+# 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

[noyau (Ping after 24h), 2016/12/12 15:48:50]

Maybe I'm confused, but this file is not iOS specific and turns the feature on.
Should it turn it off on other platforms?

[Olivier, 2016/12/12 17:55:58]

That is what I tried first, but it is not possible to test the platform in
declare_ergs.
sdefresne advice is to put true here and test the platform in the BUILD.gn file

+}