Make all DOM manipulation wait until it's safe to do so (ie. all reflows for the page have completed). Fix a race where a newly-initiated reflow would be considered complete when receiving the completion notice for the previous reflow.

src/components/script/dom/document.rs

@@ -458,6 +458,12 @@ impl Document {
             window.content_changed()
         }
     }
+
+    pub fn wait_until_safe_to_modify_dom(&self) {
+        for window in self.window.iter() {
+            window.wait_until_safe_to_modify_dom();
+        }
+    }
 }
 
 impl Traceable for Document {