Add a11y label to the Available Offline toggle

Bug: 1115025
Change-Id: If7943f16b862323a26f60e36f374b2fbcd41c3ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2525565
Commit-Queue: Austin Tankiang <austinct@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Auto-Submit: Austin Tankiang <austinct@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825270}

ui/file_manager/file_manager/main.html

@@ -368,7 +368,9 @@
       <div id="action-bar">
         <div id="pinned-toggle-wrapper" hidden>
           <span id="pinned-toggle-label">$i18n{OFFLINE_COLUMN_LABEL}</span>
-          <cr-toggle id="pinned-toggle" command="#toggle-pinned"></cr-toggle>
+          <cr-toggle id="pinned-toggle" command="#toggle-pinned"
+                     aria-labelledby="pinned-toggle-label">
+          </cr-toggle>
         </div>
         <button id="tasks" class="combobutton menu-button" menu="#tasks-menu"
                 tabindex="0" hidden