default_favicon{,_32,_64}.png are used in FaviconSource, which is used by extensions via chrome://favicon/[URL]. For example, this extension[1] will use these icons for pages that don't have their own favicons. Ideally it should be possible to serve these requests with default_favicon.icon

[1] https://chrome.google.com/webstore/detail/better-history/obciceimmggglbmelaidpjlmodcebijb/related?hl=en