[Suggestions category] Warn more about removing categories.

components/ntp_snippets/category.cc

 // 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.
 
 #include "components/ntp_snippets/category.h"
 
 #include "base/logging.h"
 
 namespace ntp_snippets {
 
 // static
 Category Category::FromKnownCategory(KnownCategories known_category) {
   return FromIDValue(static_cast<int>(known_category));
 }
 
 // static
 Category Category::FromRemoteCategory(int remote_category) {
   DCHECK_GT(remote_category, 0);
   return Category(static_cast<int>(KnownCategories::REMOTE_CATEGORIES_OFFSET) +
                   remote_category);
 }
 
 // static
 Category Category::FromIDValue(int id) {
-  DCHECK(IsValidIDValue(id)) << "Not a valid ID: " << id;
+  DCHECK(IsValidIDValue(id)) << id << " is not a valid category ID. This may "
+                                      "have been caused by removal of a local "
+                                      "KnownCategory.";
   return Category(id);
 }
 
 // static
 bool Category::IsValidIDValue(int id) {
   return (id >= 0) &&
          ((id < static_cast<int>(KnownCategories::LOCAL_CATEGORIES_COUNT) ||
            id > static_cast<int>(KnownCategories::REMOTE_CATEGORIES_OFFSET)));
 }
 
@@ skipping 17 matching lines @@
                                        const Category& right) const {
   return left.id() < right.id();
 }
 
 std::ostream& operator<<(std::ostream& os, const Category& obj) {
   os << obj.id();
   return os;
 }
 
 }  // namespace ntp_snippets