SplitSettings: Fix accessibility labels for browser settings banner

The link icon is presentational, so take it out of the tab order.
Add an aria-label to the clear button.

This is needed to get a11y browser tests to pass when we flip the
SplitSettings flag.

Bug: 950007
Test: enable Chromevox, tab through settings
Change-Id: Id5be0809de94f4107231f5a7f4d932e5cb143479
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1739392
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: May Lippert <maybelle@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: May Lippert <maybelle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684395}

chrome/browser/resources/settings/chromeos/os_settings_page/os_settings_page.html

@@ -106,12 +106,16 @@
         <div id="browserSettingsBanner" class="settings-box banner">
           <div class="start">
             <span>$i18nRaw{browserSettingsBannerText}</span>
-            <a href="chrome://settings" target="_blank">
+            <!-- Use role="presentation" because the <span> has an accessible
+                 link to settings.-->
+            <a href="chrome://settings" target="_blank"
+                tabindex="-1" role="presentation">
               <iron-icon id="openInNewBrowserSettingsIcon"
                   icon="cr:open-in-new"></iron-icon>
             </a>
           </div>
           <cr-icon-button class="icon-clear"
+              aria-label="$i18n{clear}"
               on-click="onHideBrowserSettingsBannerClick_">
           </cr-icon-button>
         </div>