Add 'X' button to 'Manage search engines' tab order.

'X' delete button is now part of the focus order in the 'Manage search
engines' options page.

BUG=276874

Review URL: https://codereview.chromium.org/666823003

Cr-Commit-Position: refs/heads/master@{#300528}

chrome/browser/resources/options/search_engine_manager_engine_list.js

@@ -111,6 +111,7 @@ cr.define('options.search_engines', function() {
         this.classList.add('default');
 
       this.deletable = engine.canBeRemoved;
+      this.closeButtonElement.tabIndex = 0;
 
       // Construct the name column.
       var nameColEl = this.ownerDocument.createElement('div');