Add gn args and BUILDFLAGS for OfflinePages, instead on relying on is_android and is_official_build.

components/offline_pages/features/features.gni

+# 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.
+
+declare_args() {
+  # Whether to enable OfflinePages support. Currently user-visible features
+  # are Android-only.
+  enable_offline_pages = is_android
+
+  # This enables test API for locally-built harness which is used for quality
+  # evaluations. Requires setting this variable manually at local environment.
+  enable_offline_pages_harness = false

[dewittj, 2017/05/11 17:03:40]

Probably want to +cc romax and petewil

[Dmitry Titov, 2017/05/11 17:35:04]

Discussed it with romax, adding him as R

[romax, 2017/05/11 18:09:21]

this flag LGTM and I think it's better getting rid off that 'is_official_build'
flag :)

+}