Fix regression: Make username not tab-focusable in passwords page.

Clicking with mouse will still expand username if it had been shortened
by ellipsis.
Username can still be focused with a screen reader.

Bug: 826654
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ia77fa1f5ac6b90a9b97102ea4724df7d9f308a41
Reviewed-on: https://chromium-review.googlesource.com/984521Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547098}

chrome/browser/resources/settings/passwords_and_forms_page/password_list_item.html

@@ -44,7 +44,7 @@
         </a>
       </div>
       <input id="username" class="username-column text-elide password-field"
-          readonly value="[[item.entry.loginPair.username]]">
+          readonly tabindex="-1" value="[[item.entry.loginPair.username]]">
       <div class="password-column">
         <template is="dom-if" if="[[!item.entry.federationText]]">
           <input id="password" aria-label=$i18n{editPasswordPasswordLabel}