Add an experimental flag for offlining.

components/offline_pages/offline_page_feature.h

Index: components/offline_pages/offline_page_feature.h
diff --git a/components/offline_pages/offline_page_feature.h b/components/offline_pages/offline_page_feature.h
index 96fd5a1dbc581963d2d5ace4ee753d0b32e5657e..bdfefaf29962ad7741068adbf83ce07c0d6ac0a5 100644
--- a/components/offline_pages/offline_page_feature.h
+++ b/components/offline_pages/offline_page_feature.h
@@ -13,6 +13,7 @@ namespace offline_pages {
 extern const base::Feature kOfflineBookmarksFeature;
 extern const base::Feature kOffliningRecentPagesFeature;
 extern const base::Feature kOfflinePagesBackgroundLoadingFeature;
+extern const base::Feature kOfflinePagesCTFeature;
 
 // Returns true if offline pages, as result of one or more offline features
 // being enabled, is enabled.
@@ -27,6 +28,9 @@ bool IsOffliningRecentPagesEnabled();
 // Returns true if saving offline pages in the background is enabled.
 bool IsOfflinePagesBackgroundLoadingEnabled();
 
+// Returns true if offline CT features are enabled.  See crbug.com/620421.
+bool IsOfflinePagesCTEnabled();
+
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_OFFLINE_PAGE_FEATURE_H_