Implement "handled for descendants" tracking for RestyleDamage.

MozReview-Commit-ID: Bbk99ogILXC

components/style/gecko_bindings/bindings.rs

@@ -649,6 +649,10 @@ extern "C" {
                                      newstyle: ServoComputedValuesBorrowed)
      -> nsChangeHint;
 }
+extern "C" {
+    pub fn Gecko_HintsHandledForDescendants(aHint: nsChangeHint)
+     -> nsChangeHint;
+}
 extern "C" {
     pub fn Gecko_CreateElementSnapshot(element: RawGeckoElementBorrowed)
      -> ServoElementSnapshotOwned;