layout: Make the LocalImageCache on_image_available callback not use

`@`.

This is one small step toward parallel layout.

src/components/main/layout/layout_task.rs

@@ -675,15 +675,15 @@ impl LayoutTask {
     // to the script task, and ultimately cause the image to be
     // re-requested. We probably don't need to go all the way back to
     // the script task for this.
-    fn make_on_image_available_cb(&self) -> @ImageResponder {
+    fn make_on_image_available_cb(&self) -> ~ImageResponder:Send {
         // This has a crazy signature because the image cache needs to
         // make multiple copies of the callback, and the dom event
         // channel is not a copyable type, so this is actually a
         // little factory to produce callbacks
-        @LayoutImageResponder {
+        ~LayoutImageResponder {
             id: self.id.clone(),
             script_chan: self.script_chan.clone(),
-        } as @ImageResponder
+        } as ~ImageResponder:Send
     }
 
     /// Handles a message to destroy layout data. Layout data must be destroyed on *this* task