A11y: Extensions: Add heading role in extensions-keyboard-shortcuts.

Bug: 876601
Change-Id: If0bd85c62c7f25b46df2ca5c5d0905d2bc9d14b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1887845Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Auto-Submit: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710572}

chrome/browser/resources/extensions/keyboard_shortcuts.html

@@ -89,7 +89,7 @@
                     item.type,
                     '$i18nPolymer{appIcon}',
                     '$i18nPolymer{extensionIcon}')]]">
-            <span>[[item.name]]</span>
+            <span role="heading" aria-level="2">[[item.name]]</span>
           </div>
           <div class="card-controls">
             <template is="dom-repeat" items="[[item.commands]]" as="command">