This enables `opener.focus()` from a document picture-in-picture window to bring system-level focus to the tab that owns the document picture-in-picture window. This allows developers to bring the original tab back to the foreground when necessary, e.g. when the user needs to access a more full UI/experience that doesn't fit in the smaller picture-in-picture window.
Returning to the opener tab of a picture-in-picture window is a common task for a user, and generally Chrome has provided a "back to tab" button in the Chrome UI for both video and document picture-in-picture. However, there are cases where websites also want to be able to programmatically return to the original tab (e.g. when the smaller form-factor of the picture-in-picture window doesn’t fit the experience the user needs).