Auto merge of #17456 - asajeffrey:script-paint-worklets-size, r=emilio

Paint worklet size should agree with Chrome.

<!-- Please describe your changes on the following line: -->

Getting our implementation to agree with Chrome about the definition of the size parameter to a paint worklet.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #17376
- [X] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17456)
<!-- Reviewable:end -->

components/layout/display_list_builder.rs

@@ -1154,10 +1154,14 @@ impl FragmentDisplayListBuilding for Fragment {
                                                        paint_worklet: &PaintWorklet,
                                                        index: usize)
     {
-        // TODO: check that this is the servo equivalent of "concrete object size".
+        // This should be the "concrete object size" of the fragment.
         // https://drafts.css-houdini.org/css-paint-api/#draw-a-paint-image
         // https://drafts.csswg.org/css-images-3/#concrete-object-size
-        let size = self.content_box().size.to_physical(style.writing_mode);
+        // Experimentally, chrome is using the size in px of the box,
+        // including padding, but not border or margin, so we follow suit.
+        // https://github.com/w3c/css-houdini-drafts/issues/417
+        let unbordered_box = self.border_box - style.logical_border_width();
+        let size = unbordered_box.size.to_physical(style.writing_mode);
         let name = paint_worklet.name.clone();
 
         // If the script thread has not added any paint worklet modules, there is nothing to do!