Injects the proxy <iframe> instance in the <body> of the current
HTML page.

@api private
function install() {
	debug( 'install()' );
	if ( iframe ) {
		uninstall();
	}

	buffered = [];

	// listen to messages sent to `window`
	event.bind( window, 'message', onmessage );

	// create the <iframe>
	iframe = document.createElement( 'iframe' );

	// set `src` and hide the iframe
	iframe.src = proxyOrigin + '/wp-admin/rest-proxy/?v=2.0#' + origin;
	iframe.style.display = 'none';

	// inject the <iframe> into the <body>
	document.body.appendChild( iframe );
}