Safety check UI: a11y: focus parent element label without tabstop

* When the user clicks the run safety check button, the parent element
  is focused, without it having a tabstop. This prevents the focus from
  getting lost and is part of a11y for safety check.
* This is a follow-up and improvement to crrev.com/c/2106089.
* DD: http://shortn/_uEauSbBMVy

Bug: 1015841
Change-Id: I5f7f418769c6908e3c7bcc3c81fe8c497ac1edb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107988
Commit-Queue: Rainhard Findling <rainhard@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752046}

chrome/browser/resources/settings/safety_check_page/safety_check_page.html

@@ -52,7 +52,8 @@
         }
       }
     </style>
-    <div id="safetyCheckParent" class="settings-box first two-line">
+    <div id="safetyCheckParent" class="settings-box first two-line"
+        tabindex="-1">
       <iron-icon icon="settings20:safety-check" aria-hidden="true">
       </iron-icon>
       <div class="start settings-box-text" no-search>

chrome/browser/resources/settings/safety_check_page/safety_check_page.js

@@ -324,10 +324,14 @@ Polymer({
     settings.HatsBrowserProxyImpl.getInstance().tryShowSurvey();
 
     this.runSafetyCheck_();
+    this.focusParent_();
+  },
 
-    // TODO(crbug.com/1015841): Prevent the focus from getting lost, once it
-    // has been decided which element to focus after the safety check
-    // parent button got clicked, and, as a result, disappeared.
+  /** @private */
+  focusParent_() {
+    const parent =
+        /** @type {!Element} */ (this.$$('#safetyCheckParent'));
+    parent.focus();
   },
 
   /**