layout: Support inline incremental reflow, and stop reconstructing all

flows when mousing over the document.

This exposes more "jumpiness" on sites like Hacker News, but the bug
that causes it was pre-existing.

components/layout/traversal.rs

@@ -170,7 +170,7 @@ impl<'a> PreorderDomTraversal for RecalcStyleForNode<'a> {
                                         &some_bf,
                                         &mut applicable_declarations,
                                         &mut shareable);
-                    } else {
+                    } else if node.has_changed() {
                         ThreadSafeLayoutNode::new(&node).set_restyle_damage(
                             incremental::rebuild_and_reflow())
                     }