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() }
     }

components/style/dom.rs

@@ -126,6 +126,10 @@ pub trait TNode : Sized + Copy + Clone + Debug + NodeInfo {
     /// Get this node's children from the perspective of a restyle traversal.
     fn traversal_children(&self) -> LayoutIterator<Self::ConcreteChildrenIterator>;
 
+    /// Returns whether `children()` and `traversal_children()` might return
+    /// iterators over different nodes.
+    fn children_and_traversal_children_might_differ(&self) -> bool;
+
     /// Converts self into an `OpaqueNode`.
     fn opaque(&self) -> OpaqueNode;
 

components/style/gecko/wrapper.rs

@@ -34,6 +34,7 @@ use gecko_bindings::bindings::{Gecko_IsRootElement, Gecko_MatchesElement, Gecko_
 use gecko_bindings::bindings::{Gecko_SetNodeFlags, Gecko_UnsetNodeFlags};
 use gecko_bindings::bindings::Gecko_ClassOrClassList;
 use gecko_bindings::bindings::Gecko_ElementHasAnimations;
+use gecko_bindings::bindings::Gecko_ElementHasBindingWithAnonymousContent;
 use gecko_bindings::bindings::Gecko_ElementHasCSSAnimations;
 use gecko_bindings::bindings::Gecko_ElementHasCSSTransitions;
 use gecko_bindings::bindings::Gecko_GetActiveLinkAttrDeclarationBlock;
@@ -271,6 +272,10 @@ impl<'ln> TNode for GeckoNode<'ln> {
         }
     }
 
+    fn children_and_traversal_children_might_differ(&self) -> bool {
+        self.as_element().map_or(false, |e| unsafe { Gecko_ElementHasBindingWithAnonymousContent(e.0) })
+    }
+
     fn opaque(&self) -> OpaqueNode {
         let ptr: usize = self.0 as *const _ as usize;
         OpaqueNode(ptr)