stylo: Don't traverse the whole dom every restyle, propagate the dirty flag down the DOM.

This commit adds hooks to the Servo style traversal to avoid traversing all the
DOM for every restyle. Additionally it changes the behavior of the dirty flag to
be propagated top down, to prevent extra overhead when an element is dirtied.

This commit doesn't aim to change the behavior on Servo just yet, since Servo
might rely on a full bottom up reconstruction of the flows. I'll need to double
check and implement that separately.

components/style/traversal.rs

@@ -147,6 +147,12 @@ pub trait DomTraversalContext<N: TNode>  {
     fn process_preorder(&self, node: N);
     /// Process `node` on the way up, after its children have been processed.
     fn process_postorder(&self, node: N);
+
+    /// Returns if the node should be processed by the preorder traversal.
+    fn should_process(&self, _node: N) -> bool { true }
+
+    /// Do an action over the child before pushing him to the work queue.
+    fn pre_process_child_hook(&self, _parent: N, _kid: N) {}
 }
 
 /// Calculates the style for a single node.
@@ -245,21 +251,15 @@ pub fn recalc_style_at<'a, N, C>(context: &'a C,
     put_thread_local_bloom_filter(bf, &unsafe_layout_node, context.shared_context());
 
     // Mark the node as DIRTY_ON_VIEWPORT_SIZE_CHANGE is it uses viewport percentage units.
-    match node.as_element() {
-        Some(element) => {
-            match *element.style_attribute() {
-                Some(ref property_declaration_block) => {
-                    if property_declaration_block.declarations().any(|d| d.0.has_viewport_percentage()) {
-                        unsafe {
-                            node.set_dirty_on_viewport_size_changed();
-                        }
-                        node.set_descendants_dirty_on_viewport_size_changed();
-                    }
-                },
-                None => {}
+    if let Some(element) = node.as_element() {
+        if let Some(ref property_declaration_block) = *element.style_attribute() {
+            if property_declaration_block.declarations().any(|d| d.0.has_viewport_percentage()) {
+                unsafe {
+                    node.set_dirty_on_viewport_size_changed();
+                }
+                node.set_descendants_dirty_on_viewport_size_changed();
             }
-        },
-        None => {}
+        }
     }
 }