Pass through visited style for after change

When a transition update is needed during restyling, `get_after_change_style` is
called to re-cascade without transition rules, and this style is then used for
primary styling instead of the style just computed.

`get_after_change_style` used to ignore visited styles, so this meant that if a
transition update was needed, visited styles were effectively dropped.

To fix the crash here, we pass through the existing visited styles when
re-cascading as part of `get_after_change_style`.

MozReview-Commit-ID: 4HBQAdeJ38B

components/style/properties/gecko.mako.rs

@@ -171,6 +171,11 @@ impl ComputedValues {
         self.rules.as_ref().unwrap()
     }
 
+    /// Whether there is a visited style.
+    pub fn has_visited_style(&self) -> bool {
+        self.visited_style.is_some()
+    }
+
     /// Gets a reference to the visited style, if any.
     pub fn get_visited_style(&self) -> Option<<&Arc<ComputedValues>> {
         self.visited_style.as_ref()