Stringify unknown JavaScript objects in global exception handlers
When turning DOM exceptions into `ErrorInfo` always try to stringify
the JavaScript value, even if it's an object that isn't a `DOMException`
or native exception. This means that exceptions that extend the `Error`
prototype are now stringified. The result is that test output for WPT
global assertion failures is more useful. For instance for the test
include-frames-from-child-same-origin-grandchild.sub.html:
Before:
```
uncaught exception: unknown (can't convert to string)
```
After:
```
uncaught exception: Error: assert_equals: expected 4 but got 3
```
---
<!-- 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] There are tests for these changes.
When turning DOM exceptions into `ErrorInfo` always try to stringify
the JavaScript value, even if it's an object that isn't a `DOMException`
or native exception. This means that exceptions that extend the `Error`
prototype are now stringified. The result is that test output for WPT
global assertion failures is more useful. For instance for the test
include-frames-from-child-same-origin-grandchild.sub.html:
Before:
```
uncaught exception: unknown (can't convert to string)
```
After:
```
uncaught exception: Error: assert_equals: expected 4 but got 3
```
WebXR Layers update WebIDL
<!-- Please describe your changes on the following line: -->
Updates the WebXR Layers WebIDL. Implements the "update the pending layers state" algorithm from the layer spec.
---
<!-- 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] There are updated 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. -->
Layout 2020: Implement basic white-space: pre support
With these changes `<pre>` and `<br>` preserve spaces and force line breaks appropriately.
---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#26440
- [x] There are tests for these changes
