Clear visited rules for text inheritance

There are two key steps when resolving text styles with `::first-line`:

1. `ResolveStyleForText` computes the initial style of the text via
   `Servo_ComputedValues_Inherit`
2. `ReparentStyleContext` is called to update style data when the first line
   of text is moved to be a child of the `::first-line` frame

Before this patch, `Servo_ComputedValues_Inherit` would clear out unvisited
rules, but visited styles (with rules inside) were cloned as-is, meaning that
step 1 might leave the text node with a style that has:

* Unvisited rules: None
* Visited rules: Some

When we later go to step 2 and re-parent onto the `::first-line` styles, we try
to cascade with these leftover visited rules.  This causes any `::first-line`
styles from our parent to be overridden by these rules which are no longer quite
right for the new frame tree.

In this patch, we resolve this by changing `StyleBuilder::for_inheritance`
(which is used by step 1's `Servo_ComputedValues_Inherit`) to also clear out
visited rules, so that we use the same logic for both unvisited and visited text
styles when reparenting onto the `::first-line` frame.

MozReview-Commit-ID: 3sgc4eGHBXs

components/style/properties/gecko.mako.rs

@@ -251,7 +251,7 @@ impl ops::DerefMut for ComputedValues {
 
 impl ComputedValuesInner {
     /// Clone the visited style.  Used for inheriting parent styles in
-    /// StyleBuilder::for_inheritance.
+    /// StyleBuilder::for_derived_style.
     pub fn clone_visited_style(&self) -> Option<Arc<ComputedValues>> {
         self.visited_style.as_ref().map(|x| x.clone_arc())
     }