style: Add TNode::children_and_traversal_children_might_differ.

This will be used to optimize out traversing the real DOM children when
propagating restyle hints.

components/script/layout_wrapper.rs

@@ -185,6 +185,12 @@ impl<'ln> TNode for ServoLayoutNode<'ln> {
         self.children()
     }
 
+    fn children_and_traversal_children_might_differ(&self) -> bool {
+        // Servo doesn't have to worry about nodes being rearranged in the
+        // flattened tree like Gecko does (for XBL and Shadow DOM).  Yet.
+        false
+    }
+
     fn opaque(&self) -> OpaqueNode {
         unsafe { self.get_jsmanaged().opaque() }
     }