Fix panics in the script_task memory reporter.

These are caused by page_root being empty.

components/script/script_task.rs

@@ -1188,7 +1188,9 @@ impl ScriptTask {
         let mut urls = vec![];
         let mut dom_tree_size = 0;
         let mut reports = vec![];
-        for it_page in self.root_page().iter() {
+
+        if let Some(root_page) = self.page.borrow().as_ref() {
+            for it_page in root_page.iter() {
                 let current_url = it_page.document().url().serialize();
                 urls.push(current_url.clone());
 
@@ -1206,6 +1208,7 @@ impl ScriptTask {
                     size: dom_tree_size,
                 })
             }
+        }
         let path_seg = format!("url({})", urls.join(", "));
         reports.extend(ScriptTask::get_reports(self.get_cx(), path_seg));
         reports_chan.send(reports);