[Suggestions category] Warn more about removing categories.

components/ntp_snippets/category.h

 // 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.
 
 #ifndef COMPONENTS_NTP_SNIPPETS_CATEGORY_H_
 #define COMPONENTS_NTP_SNIPPETS_CATEGORY_H_
 
 #include <ostream>
 
 namespace ntp_snippets {
@@ skipping 15 matching lines @@
   DOWNLOADS,
 
   // Recently used bookmarks.
   BOOKMARKS,
 
   // Physical Web page available in the vicinity.
   PHYSICAL_WEB_PAGES,
 
   // Pages recently browsed to on other devices.
   FOREIGN_TABS,
-  // INSERT NEW LOCAL CATEGORIES HERE!
+
+  // ****************** INSERT NEW LOCAL CATEGORIES HERE! ******************
+  // Existing categories are persisted and they must never be removed. This may
+  // happen implicitly, e.g. when an older version without some local category
+  // is installed.
 
   // Follows the last local category.
   LOCAL_CATEGORIES_COUNT,
 
   // Remote categories come after this.
   REMOTE_CATEGORIES_OFFSET = 10000,
 
   // Articles for you.
   ARTICLES,
-  // INSERT NEW REMOTE CATEGORIES HERE!
+
+  // ****************** INSERT NEW REMOTE CATEGORIES HERE! ******************
 
   // Tracks the last known remote category
   LAST_KNOWN_REMOTE_CATEGORY = ARTICLES,
 };
 
 // A category groups ContentSuggestions which belong together. Use the
 // CategoryFactory to obtain instances.
 class Category {
  public:
   // An arbitrary but consistent ordering. Can be used to look up categories in
@@ skipping 41 matching lines @@
 
 struct Category::CompareByID {
   bool operator()(const Category& left, const Category& right) const;
 };
 
 std::ostream& operator<<(std::ostream& os, const Category& obj);
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CATEGORY_H_