Match and cascade visited styles

To support visited styles, we match and cascade a separate set of styles any
time we notice that an element has a relevant link.

The visited rules and values are held in `ComputedStyle` alongside the
regular rules and values, which simplifies supporting various APIs like
`cascade_primary_and_pseudos` which expect easy access to previously matched
rules.

To simplify passing the additional values around, an additional reference to the
visited `ComputedValues` is placed inside the regular `ComputedValues`.

MozReview-Commit-ID: 2ebbjcfkfWf

components/style/rule_tree/mod.rs

@@ -16,6 +16,7 @@ use std::ptr;
 use std::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
 use stylearc::Arc;
 use stylesheets::StyleRule;
+use stylist::ApplicableDeclarationList;
 use thread_state;
 
 /// The rule tree, the structure servo uses to preserve the results of selector
@@ -215,6 +216,18 @@ impl RuleTree {
         current
     }
 
+    /// Given a list of applicable declarations, insert the rules and return the
+    /// corresponding rule node.
+    pub fn compute_rule_node(&self,
+                             applicable_declarations: &mut ApplicableDeclarationList,
+                             guards: &StylesheetGuards)
+                             -> StrongRuleNode
+    {
+        let rules = applicable_declarations.drain().map(|d| (d.source, d.level));
+        let rule_node = self.insert_ordered_rules_with_important(rules, guards);
+        rule_node
+    }
+
     /// Insert the given rules, that must be in proper order by specifity, and
     /// return the corresponding rule node representing the last inserted one.
     pub fn insert_ordered_rules<'a, I>(&self, iter: I) -> StrongRuleNode
@@ -632,6 +645,8 @@ struct WeakRuleNode {
 /// A strong reference to a rule node.
 #[derive(Debug, PartialEq)]
 pub struct StrongRuleNode {
+    // TODO: Mark this as NonZero once stable to save space inside Option.
+    // https://github.com/rust-lang/rust/issues/27730
     ptr: *mut RuleNode,
 }