[WebUI] Remove destructuring assignment usage

Destructuring assignment is not currently allowed in Chrome WebUI, per
the ES6 styleguide:
https://chromium.googlesource.com/chromium/src/+/master/docs/es6_chromium.md#Destructuring-Assignment

Bug: None
Change-Id: I000da1a21e148fe9e8024202fb0e5a6f1bca3cc8
Reviewed-on: https://chromium-review.googlesource.com/1142566Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576329}

chrome/test/data/webui/settings/privacy_page_test.js

@@ -186,7 +186,9 @@ cr.define('settings_privacy_page', function() {
         actionButton.click();
 
         return testBrowserProxy.whenCalled('clearBrowsingData')
-            .then(function([dataTypes, timePeriod]) {
+            .then(function(args) {
+              const dataTypes = args[0];
+              const timePeriod = args[1];
               assertEquals(1, dataTypes.length);
               assertEquals('browser.clear_data.cookies_basic', dataTypes[0]);
               assertTrue(element.$$('#clearBrowsingDataDialog').open);