script: Fix a few load related bugs.

This is what was making me hit the new test failures. So turns out that when the
DOMContentLoaded event is fired we fired no messages to the constellation, but
we fired the DOMLoad message from the DocumentProgressHandler, effectively after
having dispatched the Load message from script thread.

This also fixes the possibility of a subframe navigation not blocking the load
event of the parent document, for example.

components/compositing/compositor.rs

@@ -2114,7 +2114,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 }
 
                 // Check if there are any pending frames. If so, the image is not stable yet.
-                if self.pending_subpages.len() > 0 {
+                if !self.pending_subpages.is_empty() {
                     return Err(NotReadyToPaint::PendingSubpages(self.pending_subpages.len()));
                 }