Auto merge of #16164 - heycam:stale-styles, r=bholley

stylo: Add argument to Servo_ResolveStyle to allow stale styles to be returned.

This is the Servo-side part of https://bugzilla.mozilla.org/show_bug.cgi?id=1350671.

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components/style/gecko_bindings/bindings.rs

@@ -1812,7 +1812,8 @@ extern "C" {
 }
 extern "C" {
     pub fn Servo_ResolveStyle(element: RawGeckoElementBorrowed,
-                              set: RawServoStyleSetBorrowed)
+                              set: RawServoStyleSetBorrowed,
+                              allow_stale: bool)
      -> ServoComputedValuesStrong;
 }
 extern "C" {

ports/geckolib/glue.rs

@@ -1473,15 +1473,22 @@ pub extern "C" fn Servo_TakeChangeHint(element: RawGeckoElementBorrowed) -> nsCh
 
 #[no_mangle]
 pub extern "C" fn Servo_ResolveStyle(element: RawGeckoElementBorrowed,
-                                     raw_data: RawServoStyleSetBorrowed)
+                                     raw_data: RawServoStyleSetBorrowed,
+                                     allow_stale: bool)
                                      -> ServoComputedValuesStrong
 {
     let element = GeckoElement(element);
     debug!("Servo_ResolveStyle: {:?}", element);
     let data = unsafe { element.ensure_data() }.borrow_mut();
 
-    if !data.has_current_styles() {
+    let valid_styles = if allow_stale {
-        warn!("Resolving style on unstyled element with lazy computation forbidden.");
+      data.has_styles()
+    } else {
+      data.has_current_styles()
+    };
+
+    if !valid_styles {
+        warn!("Resolving style on element without current styles with lazy computation forbidden.");
         let per_doc_data = PerDocumentStyleData::from_ffi(raw_data).borrow();
         return per_doc_data.default_computed_values().clone().into_strong();
     }