style: Allow styles to be computed ignoring existing element data.

components/layout/query.rs

@@ -702,7 +702,7 @@ pub fn process_resolved_style_request<'a, N>(context: &LayoutContext,
         thread_local: &mut tlc,
     };
 
-    let styles = resolve_style(&mut context, element, RuleInclusion::All);
+    let styles = resolve_style(&mut context, element, RuleInclusion::All, false);
     let style = styles.primary();
     let longhand_id = match *property {
         PropertyId::Longhand(id) => id,

components/style/gecko/generated/bindings.rs

@@ -2810,7 +2810,8 @@ extern "C" {
                                     rule_inclusion: StyleRuleInclusion,
                                     snapshots:
                                         *const ServoElementSnapshotTable,
-                                    set: RawServoStyleSetBorrowed)
+                                    set: RawServoStyleSetBorrowed,
+                                    ignore_existing_styles: bool)
      -> ServoStyleContextStrong;
 }
 extern "C" {

components/style/traversal.rs

@@ -388,6 +388,7 @@ pub fn resolve_style<E>(
     context: &mut StyleContext<E>,
     element: E,
     rule_inclusion: RuleInclusion,
+    ignore_existing_style: bool,
 ) -> ElementStyles
 where
     E: TElement,
@@ -395,6 +396,7 @@ where
     use style_resolver::StyleResolverForElement;
 
     debug_assert!(rule_inclusion == RuleInclusion::DefaultOnly ||
+                  ignore_existing_style ||
                   element.borrow_data().map_or(true, |d| !d.has_styles()),
                   "Why are we here?");
     let mut ancestors_requiring_style_resolution = SmallVec::<[E; 16]>::new();
@@ -405,7 +407,7 @@ where
     let mut style = None;
     let mut ancestor = element.traversal_parent();
     while let Some(current) = ancestor {
-        if rule_inclusion == RuleInclusion::All {
+        if rule_inclusion == RuleInclusion::All && !ignore_existing_style {
             if let Some(data) = current.borrow_data() {
                 if let Some(ancestor_style) = data.styles.get_primary() {
                     style = Some(ancestor_style.clone());