Replace DisplayList::is_contentful with tracking during conversion to WR display lists

components/layout_2020/display_list/items.rs

@@ -3,7 +3,6 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use euclid::Vector2D;
-use gfx_traits;
 use std::collections::HashMap;
 use std::f32;
 use webrender_api::units::LayoutPixel;
@@ -14,11 +13,5 @@ pub use style::dom::OpaqueNode;
 #[derive(Serialize)]
 pub struct DisplayList {}
 
-impl gfx_traits::DisplayList for DisplayList {
-    fn is_contentful(&self) -> bool {
-        false
-    }
-}
-
 /// The type of the scroll offset list. This is only populated if WebRender is in use.
 pub type ScrollOffsetMap = HashMap<ExternalScrollId, Vector2D<f32, LayoutPixel>>;

components/layout_2020/display_list/webrender_helpers.rs

@@ -7,8 +7,17 @@ use msg::constellation_msg::PipelineId;
 use webrender_api::units::LayoutSize;
 use webrender_api::{self, DisplayListBuilder};
 
+/// Contentful paint, for the purpose of
+/// https://w3c.github.io/paint-timing/#first-contentful-paint
+/// (i.e. the display list contains items of type text,
+/// image, non-white canvas or SVG). Used by metrics.
+pub struct IsContentful(pub bool);
+
 impl DisplayList {
-    pub fn convert_to_webrender(&mut self, pipeline_id: PipelineId) -> DisplayListBuilder {
+    pub fn convert_to_webrender(
+        &mut self,
+        pipeline_id: PipelineId,
+    ) -> (DisplayListBuilder, IsContentful) {
         let webrender_pipeline = pipeline_id.to_webrender();
 
         let builder = DisplayListBuilder::with_capacity(
@@ -17,6 +26,8 @@ impl DisplayList {
             1024 * 1024, // 1 MB of space
         );
 
-        builder
+        let is_contentful = IsContentful(false);
+
+        (builder, is_contentful)
     }
 }