Add more a11y description to the Drive Offline banner

This change makes the screenreader read both the banner text and the
button text when focused on the "Learn more" button.

Bug: 1129778
Change-Id: Id2953e7216edd2b4d9bd763dc8ba4805fa5d1f0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533062
Commit-Queue: Austin Tankiang <austinct@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827080}

ui/file_manager/file_manager/main.html

@@ -545,9 +545,13 @@
               </div>
               <div class="volume-warning" id="offline-info-banner" hidden>
                 <div class="info-icon"></div>
-                <div class="body2-primary">$i18n{OFFLINE_BANNER_MESSAGE}</div>
+                <div id="offline-info-banner-text" class="body2-primary">
+                  $i18n{OFFLINE_BANNER_MESSAGE}
+                </div>
                 <div class="button-group">
                   <cr-button id="offline-learn-more" class="banner-close text-button"
+                             aria-labelledby="offline-info-banner-text"
+                             aria-describedby="offline-learn-more"
                              aria-disabled="false" role="button" tabindex="0">
                     $i18n{DRIVE_LEARN_MORE}
                   </cr-button>