Print Preview: Make getChildElement return required HTMLElement

chrome/browser/resources/print_preview/component.js

Index: chrome/browser/resources/print_preview/component.js
diff --git a/chrome/browser/resources/print_preview/component.js b/chrome/browser/resources/print_preview/component.js
index 3c18829a22e2e16a445447b69bc77a467f05dc62..0b53d1e875ad412ef6c040c9b86d4b40fc6e72e2 100644
--- a/chrome/browser/resources/print_preview/component.js
+++ b/chrome/browser/resources/print_preview/component.js
@@ -162,12 +162,10 @@ cr.define('print_preview', function() {
      * @param {string} query Selector query to select an element starting from
      *     the component's root element using a depth first search for the first
      *     element that matches the query.
-     * @return {HTMLElement} Element selected by the given query.
-     * TODO(alekseys): Check all call sites and rename this function to
-     *     something like getRequiredChildElement.
+     * @return {!HTMLElement} Element selected by the given query.
      */
     getChildElement: function(query) {
-      return this.element_.querySelector(query);
+      return assert(this.element_.querySelector(query));
     },
 
     /**