Make the I Tried star appear when a top-level page load fails for network-related reasons.

Under the hood, this requires treating the I Tried pipeline as a new load instead of a replacement, since the failure-handling code interacts poorly with the rest of the replacement code when we get a series of staggered failures over time from the various pipeline components.

src/components/script/script_task.rs

@@ -172,7 +172,7 @@ impl PageTree {
                 next_subpage_id: Untraceable::new(RefCell::new(SubpageId(0))),
                 resize_event: Untraceable::new(RefCell::new(None)),
                 fragment_node: Traceable::new(RefCell::new(None)),
-                last_reflow_id: Traceable::new(RefCell::new(0))
+                last_reflow_id: Traceable::new(RefCell::new(0)),
             }),
             inner: ~[],
         }