Add ClearHistory() to ContentSuggestionsService and its providers

chrome/browser/ntp_snippets/content_suggestions_service_factory.cc

Index: chrome/browser/ntp_snippets/content_suggestions_service_factory.cc
diff --git a/chrome/browser/ntp_snippets/content_suggestions_service_factory.cc b/chrome/browser/ntp_snippets/content_suggestions_service_factory.cc
index 401cda05023307225aae7cff9be8bc125ff23c57..840d55992ea6a000f34cee92ce33b78dfb8f83f2 100644
--- a/chrome/browser/ntp_snippets/content_suggestions_service_factory.cc
+++ b/chrome/browser/ntp_snippets/content_suggestions_service_factory.cc
@@ -246,7 +246,9 @@ KeyedService* ContentSuggestionsServiceFactory::BuildServiceInstanceFor(
   }
 #endif  // OS_ANDROID
 
-  if (base::FeatureList::IsEnabled(ntp_snippets::kBookmarkSuggestionsFeature)) {
+  // |bookmark_model| can be null in tests.
+  if (base::FeatureList::IsEnabled(ntp_snippets::kBookmarkSuggestionsFeature) &&
+      bookmark_model) {
     RegisterBookmarkProvider(bookmark_model, service, category_factory,
                              pref_service);
   }