[NTP::PhysicalWeb] Implement suggestion dismissal.

components/ntp_snippets/content_suggestions_service.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/content_suggestions_service.h"
 
 #include <algorithm>
 #include <iterator>
 #include <set>
 #include <utility>
@@ skipping 149 matching lines @@
 void ContentSuggestionsService::DismissSuggestion(
     const ContentSuggestion::ID& suggestion_id) {
   if (!providers_by_category_.count(suggestion_id.category())) {
     LOG(WARNING) << "Dismissed suggestion " << suggestion_id
                  << " for unavailable category " << suggestion_id.category();
     return;
   }
   providers_by_category_[suggestion_id.category()]->DismissSuggestion(
       suggestion_id);
 
-  // Remove the suggestion locally.
-  bool removed = RemoveSuggestionByID(suggestion_id);
-  DCHECK(removed) << "The dismissed suggestion " << suggestion_id
-                  << " has already been removed. Providers must not call"
-                  << " OnNewSuggestions in response to DismissSuggestion.";
+  // Remove the suggestion locally if it is present. A suggestion may be missing
+  // localy e.g. if it was sent to UI through |Fetch| or it has been dismissed
+  // from a different NTP.
+  RemoveSuggestionByID(suggestion_id);
 }
 
 void ContentSuggestionsService::DismissCategory(Category category) {
   auto providers_it = providers_by_category_.find(category);
   if (providers_it == providers_by_category_.end()) {
     return;
   }
 
   ContentSuggestionsProvider* provider = providers_it->second;
   UnregisterCategory(category, provider);
@@ skipping 310 matching lines @@
 void ContentSuggestionsService::StoreDismissedCategoriesToPrefs() {
   base::ListValue list;
   for (const auto& category_provider_pair : dismissed_providers_by_category_) {
     list.AppendInteger(category_provider_pair.first.id());
   }
 
   pref_service_->Set(prefs::kDismissedCategories, list);
 }
 
 }  // namespace ntp_snippets