[remote suggestions] Add separte fetch interval for NTP open trigger

components/ntp_snippets/pref_names.h

Index: components/ntp_snippets/pref_names.h
diff --git a/components/ntp_snippets/pref_names.h b/components/ntp_snippets/pref_names.h
index 7ff783d8a24cb7f2de241fbb021127d7a93ba764..ee604861932495f88f8ae3df6d9d1c71f470f1fb 100644
--- a/components/ntp_snippets/pref_names.h
+++ b/components/ntp_snippets/pref_names.h
@@ -20,8 +20,11 @@ extern const char kRemoteSuggestionCategories[];
 extern const char kSnippetLastFetchAttempt[];
 // The pref name for the currently applied minimal interval between two
 // successive soft background fetches that react to user activity (such as
-// opening an NTP).
+// opening Chrome).
 extern const char kSnippetSoftFetchingIntervalOnUsageEvent[];
+// The pref name for the currently applied minimal interval between two
+// successive soft brackground fetches when the New Tab Page is opened.
+extern const char kSnippetSoftFetchingIntervalOnNtpOpened[];
 
 // The pref name for the currently-scheduled background fetching interval when
 // there is WiFi connectivity.