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/context.rs

@@ -297,6 +297,11 @@ impl CascadeInputs {
         self.visited_rules.take()
     }
 
+    /// Whether there are any visited values.
+    pub fn has_visited_values(&self) -> bool {
+        self.visited_values.is_some()
+    }
+
     /// Gets a reference to the visited computed values. Panic if the element
     /// does not have visited computed values.
     pub fn visited_values(&self) -> &Arc<ComputedValues> {