Use the correct .png extension for the image placeholder.

This was changed to the mozilla broken image resource
without changing the filename, even though the file
type changed.

components/net/image_cache_task.rs

@@ -460,7 +460,7 @@ pub fn new_image_cache_task(resource_task: ResourceTask) -> ImageCacheTask {
 
         // Preload the placeholder image, used when images fail to load.
         let mut placeholder_path = resources_dir_path();
-        placeholder_path.push("rippy.jpg");
+        placeholder_path.push("rippy.png");
 
         let mut image_data = vec![];
         let result = File::open(&placeholder_path).and_then(|mut file| {

tests/wpt/mozilla/tests/css/no-image-ref.html

@@ -1 +1 @@
-<img width="100px" height="100px" src="rippy.jpg">
+<img width="100px" height="100px" src="rippy.png">