Bumps [webxr](https://github.com/servo/webxr) from `e507076` to `88f287d`.
- [Release notes](https://github.com/servo/webxr/releases)
- [Commits](servo/webxr@e507076...88f287d)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>