The proxy <iframe> instance's "load" event callback function.

@api private
function onload() {
	debug( 'proxy <iframe> "load" event' );
	loaded = true;

	// flush any buffered API calls
	if ( buffered ) {
		for ( let i = 0; i < buffered.length; i++ ) {
			submitRequest( buffered[ i ] );
		}
		buffered = null;
	}
}