Add a Stylo API for reparenting a given style.

Servo side of part 4 of the fix for Gecko bug 1324619.  r=emilio

components/style/gecko/data.rs

@@ -204,6 +204,15 @@ impl PerDocumentStyleDataImpl {
     pub fn clear_stylist(&mut self) {
         self.stylist.clear();
     }
+
+    /// Returns whether visited links are enabled.
+    fn visited_links_enabled(&self) -> bool {
+        unsafe { bindings::Gecko_AreVisitedLinksEnabled() }
+    }
+    /// Returns whether visited styles are enabled.
+    pub fn visited_styles_enabled(&self) -> bool {
+        self.visited_links_enabled() && !self.is_private_browsing_enabled()
+    }
 }
 
 unsafe impl HasFFI for PerDocumentStyleData {

components/style/gecko/generated/bindings.rs

@@ -2820,6 +2820,16 @@ extern "C" {
                                     set: RawServoStyleSetBorrowed)
      -> ServoStyleContextStrong;
 }
+extern "C" {
+    pub fn Servo_ReparentStyle(style_to_reparent: ServoStyleContextBorrowed,
+                               parent_style: ServoStyleContextBorrowed,
+                               parent_style_ignoring_first_line:
+                                   ServoStyleContextBorrowed,
+                               layout_parent_style: ServoStyleContextBorrowed,
+                               element: RawGeckoElementBorrowedOrNull,
+                               set: RawServoStyleSetBorrowed)
+     -> ServoStyleContextStrong;
+}
 extern "C" {
     pub fn Servo_TraverseSubtree(root: RawGeckoElementBorrowed,
                                  set: RawServoStyleSetBorrowed,