Add DomIf export in polymer.js

This is imported by settings_animated_pages, which will be migrated
to Polymer 3 in a followup.

Bug: 1026426
Change-Id: If0808e2b900f869a6bc317b509be502bf5b836bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036928Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738315}

third_party/polymer/v3_0/components-chromium/polymer/polymer_bundled.js

@@ -15020,4 +15020,4 @@ 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 { Base, Debouncer, OptionalMutableDataBehavior, Polymer, PolymerElement, TemplateInstanceBase, Templatizer, afterNextRender, animationFrame, beforeNextRender, calculateSplices, dashToCamelCase, dom, enqueueDebouncer, flush, gestures$1 as gestures, html, idlePeriod, matches, microTask, templatize, translate, useShadow };
+export { Base, Debouncer, DomIf, OptionalMutableDataBehavior, Polymer, PolymerElement, TemplateInstanceBase, Templatizer, afterNextRender, animationFrame, beforeNextRender, calculateSplices, dashToCamelCase, dom, enqueueDebouncer, flush, gestures$1 as gestures, html, idlePeriod, matches, microTask, templatize, translate, useShadow };

third_party/polymer/v3_0/polymer.js

@@ -16,6 +16,7 @@ export {Base} from './polymer-legacy.js';
 export {dashToCamelCase} from './lib/utils/case-map.js';
 export {Debouncer, enqueueDebouncer} from './lib/utils/debounce.js';
 export {dom, flush} from './lib/legacy/polymer.dom.js';
+export {DomIf} from './lib/elements/dom-if.js';
 export {html} from './lib/utils/html-tag.js';
 export {matches, translate} from './lib/utils/path.js';
 export {OptionalMutableDataBehavior} from './lib/legacy/mutable-data-behavior.js';