function nextFrame() {
	  return new Promise(function (resolve) {
	    return delayCallback(function () {
	      return resolve();
	    });
	  });
	}
Returns a promise that resolve 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'}