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/data.rs

@@ -37,13 +37,13 @@ impl PrivateStyleData {
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct DomParallelInfo {
     /// The number of children that still need work done.
-    pub children_count: AtomicIsize,
+    pub children_to_process: AtomicIsize,
 }
 
 impl DomParallelInfo {
     pub fn new() -> DomParallelInfo {
         DomParallelInfo {
-            children_count: AtomicIsize::new(0),
+            children_to_process: AtomicIsize::new(0),
         }
     }
 }