Handles a "progress" event being proxied back from the iframe page.

@param {Object} data
@private
function onprogress( data ) {
	debug( 'got "progress" event: %o', data );
	const xhr = requests[ data.callbackId ];
	if ( xhr ) {
		const prog = new ProgressEvent( 'progress', data );
		const target = data.upload ? xhr.upload : xhr;
		target.dispatchEvent( prog );
	}
}