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/selectors/matching.rs

@@ -75,7 +75,7 @@ impl ElementSelectorFlags {
 ///
 /// There are two modes of selector matching. The difference is only noticeable
 /// in presence of pseudo-elements.
-#[derive(Debug, PartialEq)]
+#[derive(Debug, PartialEq, Copy, Clone)]
 pub enum MatchingMode {
     /// Don't ignore any pseudo-element selectors.
     Normal,
@@ -108,13 +108,14 @@ pub enum VisitedHandlingMode {
 /// Data associated with the matching process for a element.  This context is
 /// used across many selectors for an element, so it's not appropriate for
 /// transient data that applies to only a single selector.
+#[derive(Clone)]
 pub struct MatchingContext<'a> {
     /// Output that records certains relations between elements noticed during
     /// matching (and also extended after matching).
     pub relations: StyleRelations,
-    /// The matching mode we should use when matching selectors.
+    /// Input with the matching mode we should use when matching selectors.
     pub matching_mode: MatchingMode,
-    /// The bloom filter used to fast-reject selectors.
+    /// Input with the bloom filter used to fast-reject selectors.
     pub bloom_filter: Option<&'a BloomFilter>,
     /// Input that controls how matching for links is handled.
     pub visited_handling: VisitedHandlingMode,
@@ -122,7 +123,7 @@ pub struct MatchingContext<'a> {
     /// matching _any_ selector for this element. (This differs from
     /// `RelevantLinkStatus` which tracks the status for the _current_ selector
     /// only.)
-    relevant_link_found: bool,
+    pub relevant_link_found: bool,
 }
 
 impl<'a> MatchingContext<'a> {
@@ -139,6 +140,21 @@ impl<'a> MatchingContext<'a> {
             relevant_link_found: false,
         }
     }
+
+    /// Constructs a new `MatchingContext` for use in visited matching.
+    pub fn new_for_visited(matching_mode: MatchingMode,
+                           bloom_filter: Option<&'a BloomFilter>,
+                           visited_handling: VisitedHandlingMode)
+                           -> Self
+    {
+        Self {
+            relations: StyleRelations::empty(),
+            matching_mode: matching_mode,
+            bloom_filter: bloom_filter,
+            visited_handling: visited_handling,
+            relevant_link_found: false,
+        }
+    }
 }
 
 pub fn matches_selector_list<E>(selector_list: &[Selector<E::Impl>],