dillon1000/react

React.Timeout -> React.Placeholder (#13105)

Changed the API to match what we've been using in our latest discussions.

Our tentative plans are for <Placeholder> to automatically hide the timed-out
children, instead of removing them, so their state is not lost. This part is
not yet implemented. We'll likely have a lower level API that does not include
the hiding behavior. This is also not yet implemented.