Set `detail` to null by default to comply with the spec

[hoxyq]

Summary:

Changelog: [Internal]

This is required by the spec:

• performance.mark: https://w3c.github.io/user-timing/#the-performancemark-constructor, see step 7.
• performance.measure: https://w3c.github.io/user-timing/#dom-performance-measure, see step 9.

Differential Revision: D75611976

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D75611976