Calls the `postMessage()` function on the <iframe>.

@param {Object} params
@api private
function submitRequest( params ) {
	debug( 'sending API request to proxy <iframe> %o', params );

	// `formData` needs to be patched if it contains `File` objects to work around
	// a Chrome bug. See `patchFileObjects` description for more details.
	if ( params.formData ) {
		patchFileObjects( params.formData );
	}

	iframe.contentWindow.postMessage( postStrings ? JSON.stringify( params ) : params, proxyOrigin );
}