Remove deleted offline page suggestions from opened NTPs

components/ntp_snippets/content_suggestions_provider.h

Index: components/ntp_snippets/content_suggestions_provider.h
diff --git a/components/ntp_snippets/content_suggestions_provider.h b/components/ntp_snippets/content_suggestions_provider.h
index 23d403c44dc5012e025e00df10fac45e96e07e5a..836fd41996ddd1d2589959e73426e70f72194709 100644
--- a/components/ntp_snippets/content_suggestions_provider.h
+++ b/components/ntp_snippets/content_suggestions_provider.h
@@ -64,6 +64,19 @@ class ContentSuggestionsProvider {
     virtual void OnCategoryStatusChanged(ContentSuggestionsProvider* provider,
                                          Category category,
                                          CategoryStatus new_status) = 0;
+
+    // Called when a suggestion has been invalidated. It will not be provided
+    // through |OnNewSuggestions| anymore, is not supported by
+    // |FetchSuggestionImage| or |DismissSuggestion| anymore, and should
+    // immediately be cleared from the UI and caches. This happens, for example,
+    // when the content that the suggestion refers to is gone.
+    // Note that this event may be fired even if the corresponding |category| is
+    // not currently AVAILABLE, because open UIs may still be showing the
+    // suggestion that is to be removed. This event may also be fired for
+    // |suggestion_id|s that never existed and should be ignored in that case.
+    virtual void OnSuggestionInvalidated(ContentSuggestionsProvider* provider,
+                                         Category category,
+                                         const std::string& suggestion_id) = 0;
   };
 
   virtual ~ContentSuggestionsProvider();