Make Runnable::handler take self by value.

With my thanks to kimundi and eddyb for their help in making this approach
work.

components/script/dom/xmlhttprequest.rs

@@ -85,8 +85,9 @@ impl XHRProgressHandler {
 }
 
 impl Runnable for XHRProgressHandler {
-    fn handler(&self) {
-        XMLHttpRequest::handle_progress(self.addr.clone(), self.progress.clone());
+    fn handler(self: Box<XHRProgressHandler>) {
+        let this = *self;
+        XMLHttpRequest::handle_progress(this.addr, this.progress);
     }
 }