cr-checkbox can hide the ARIA description element

cr-checkbox uses a hidden element as the target of the aria-describedby
attribute. However, while that element is hidden visually it's still
part of the accessibility tree so screen reader users perceive two
different elements - the checkbox, followed by the description.

To fix this, we can just hide the description with aria-hidden=true. Its
text is still computed correctly by aria-describedby.

Bug: 1101148
Change-Id: I213d2164dad6f0f4a9caa194b16b14e7a4b85580
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466060Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816389}

ui/webui/resources/cr_elements/cr_checkbox/cr_checkbox.html

@@ -161,7 +161,7 @@ List of customizable styles:
     <div id="label-container" aria-hidden="true" part="label-container">
       <slot></slot>
     </div>
-    <div id="ariaDescription">[[ariaDescription]]</div>
+    <div id="ariaDescription" aria-hidden="true">[[ariaDescription]]</div>
   </template>
   <script src="cr_checkbox.js"></script>
 </dom-module>