Use bookmark creation date fallback for 6 weeks after installing M54

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 587ca18a92e9f7fd1608ec0bfd1e7c4f4bebe36a..8153cac627e42ebeb1f7345a7f00e3a2f33b5823 100644
--- a/components/ntp_snippets/pref_names.h
+++ b/components/ntp_snippets/pref_names.h
@@ -32,6 +32,9 @@ extern const char kSnippetThumbnailsRequestsDay[];
 extern const char kDismissedRecentOfflineTabSuggestions[];
 extern const char kDismissedDownloadSuggestions[];
 
+// The pref name for the time when M54 was first started on the device.
+extern const char kBookmarksFirstM54Start[];
+
 }  // namespace prefs
 }  // namespace ntp_snippets