[Fixtures] The removed ReactDOM.unstable_deferredUpdates method should be replaced in the fiber-triangle fixture

[mpelekh]

The error occurs when the "fiber-triangle" fixture is running.

The root cause is as follows: the removed ReactDOM.unstable_deferredUpdates() method is used to defer updates.

The requestIdleCallback() doesn't prevent the page stuttering when is used instead of the ReactDOM.unstable_deferredUpdates() (as was suggested at #13488 (comment)). This behavior could be tested at https://codesandbox.io/s/317vypvo9p

Notes: React v 16.8.3

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