Add enable-offlining-recent-pages feature switch.

components/offline_pages/offline_page_feature.h

 // Copyright 2015 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_OFFLINE_PAGES_OFFLINE_PAGE_FEATURE_H_
 #define COMPONENTS_OFFLINE_PAGES_OFFLINE_PAGE_FEATURE_H_
 
+#include "base/feature_list.h"
 #include "build/build_config.h"
 
-#if defined(OS_ANDROID)
+namespace offline_pages {
 
-namespace offline_pages {
+extern const base::Feature kOffliningRecentPagesFeature;
 
 // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.components.offlinepages
 enum class FeatureMode {
   // Offline pages feature is disabled.
   DISABLED,
   // Offline pages feature is enabled, showing bookmarks in UI strings.
   ENABLED_AS_BOOKMARKS,
   // Offline pages feature is enabled, showing saved pages in UI strings.
   ENABLED_AS_SAVED_PAGES
 };
 
 // Returns the mode where Offline Pages feature is running.
 FeatureMode GetOfflinePageFeatureMode();
 
 // Returns true if offline pages is enabled.
 bool IsOfflinePagesEnabled();
 
+// Returns true if offlining of recent pages (aka 'Last N pages') is enabled.
+bool IsOffliningRecentPagesEnabled();
+
 }  // namespace offline_pages
 
-#endif  // defined(OS_ANDROID)
-
 #endif  // COMPONENTS_OFFLINE_PAGES_OFFLINE_PAGE_FEATURE_H_