function nextFrame() {
    return new Promise((resolve)=>delayCallback(()=>resolve()));
}
Returns a promise that resolves when a requestAnimationFrame has completed.

On Node.js this uses setImmediate instead of requestAnimationFrame.

This is simply a sugar method so that users can do the following:
`await tf.nextFrame();`

@doc {heading: 'Performance', subheading: 'Timing'}