Using FF broken image icon.

Got it from:
https://mxr.mozilla.org/mozilla-central/source/layout/generic/broken-image.png

components/net/image_cache_task.rs

@@ -129,7 +129,6 @@ impl ImageCache {
     // Used to preload the default placeholder.
     fn init(&mut self) {
         let mut placeholder_url = resources_dir_path();
-        // TODO (Savago): replace for a prettier one.
         placeholder_url.push("rippy.jpg");
         let image = load_image_data(Url::from_file_path(&*placeholder_url).unwrap(), self.resource_task.clone(), &self.placeholder_data);