WebUI Polymer3: Export beforeNextRender, afterNextRender from bundle.

Bug: 965770
Change-Id: I1484770d101afca91562db6757b37ff8019ea28f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693165
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675752}

third_party/polymer/v3_0/polymer.js

@@ -8,6 +8,7 @@ Code distributed by Google as part of the polymer project is also
 subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 */
 
+export {afterNextRender, beforeNextRender} from './lib/utils/render-status.js';
 export {animationFrame, idlePeriod, microTask} from './lib/utils/async.js';
 import * as gestures from './lib/utils/gestures.js';
 export {gestures};