Auto merge of #14300 - bholley:restyle_driven_traversal, r=emilio

stylo: Basic infrastructure for RestyleHint-driven traversal

Gecko Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=131701

(Don't review yet, will flag on the gecko bug when the time comes)

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components/script/dom/document.rs

@@ -434,7 +434,6 @@ impl Document {
         // that workable.
         match self.GetDocumentElement() {
             Some(root) => {
-                root.upcast::<Node>().is_dirty() ||
                 root.upcast::<Node>().has_dirty_descendants() ||
                 !self.pending_restyles.borrow().is_empty() ||
                 self.needs_paint()

components/script/dom/node.rs

@@ -149,9 +149,6 @@ bitflags! {
         #[doc = "Specifies whether this node is in a document."]
         const IS_IN_DOC = 0x01,
         #[doc = "Specifies whether this node needs style recalc on next reflow."]
-        const IS_DIRTY = 0x04,
-        #[doc = "Specifies whether this node has descendants (inclusive of itself) which \
-                 have changed since the last reflow."]
         const HAS_DIRTY_DESCENDANTS = 0x08,
         // TODO: find a better place to keep this (#4105)
         // https://critic.hoppipolla.co.uk/showcomment?chain=8873
@@ -172,7 +169,7 @@ bitflags! {
 
 impl NodeFlags {
     pub fn new() -> NodeFlags {
-        IS_DIRTY
+        NodeFlags::empty()
     }
 }
 
@@ -428,14 +425,6 @@ impl Node {
         self.flags.set(flags);
     }
 
-    pub fn is_dirty(&self) -> bool {
-        self.get_flag(IS_DIRTY)
-    }
-
-    pub fn set_is_dirty(&self, state: bool) {
-        self.set_flag(IS_DIRTY, state)
-    }
-
     pub fn has_dirty_descendants(&self) -> bool {
         self.get_flag(HAS_DIRTY_DESCENDANTS)
     }