Add ClearHistory() to ContentSuggestionsService and its providers

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 c7fcd056abacc51b0d30dabb2bef7d3d5ace1d67..c07d702b5a6498a954b8e124e859341f90b2de3f 100644
--- a/components/ntp_snippets/content_suggestions_provider.h
+++ b/components/ntp_snippets/content_suggestions_provider.h
@@ -102,6 +102,9 @@ class ContentSuggestionsProvider {
   virtual void FetchSuggestionImage(const std::string& suggestion_id,
                                     const ImageFetchedCallback& callback) = 0;
 
+  // Removes user history (for example dismissed suggestions).

[Marc Treib, 2016/08/29 09:43:11]

I think this deserves a bit more explanation - with this text, I'm wondering how
it is different from ClearDismissedSuggestions. (Maybe dismissed suggestions
aren't the best example?)
What are the implications when this is called? E.g. providers should immediately
clear any history-related suggestions from the UI, that kind of thing.

[vitaliii, 2016/08/29 10:37:25]

Done.

+  virtual void ClearHistory() = 0;
+
   // Clears all caches for the given category, so that the next fetch starts
   // from scratch.
   virtual void ClearCachedSuggestions(Category category) = 0;