Add aria-label for expand and collapse toggle on comment

Change-Id: Ie847625dc81347948cdea98ba2d8ca78dce86861
diff --git a/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.js b/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.js
index 153b98d..6697880 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.js
+++ b/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.js
@@ -347,6 +347,10 @@
     return collapsed ? 'gr-icons:expand-more' : 'gr-icons:expand-less';
   }
 
+  _computeShowHideAriaLabel(collapsed) {
+    return collapsed ? 'Expand' : 'Collapse';
+  }
+
   _calculateActionstoShow(showActions, isRobotComment) {
     // Polymer 2: check for undefined
     if ([showActions, isRobotComment].some(arg => arg === undefined)) {
diff --git a/polygerrit-ui/app/elements/shared/gr-comment/gr-comment_html.js b/polygerrit-ui/app/elements/shared/gr-comment/gr-comment_html.js
index 3d27042..b83cdf2 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment/gr-comment_html.js
+++ b/polygerrit-ui/app/elements/shared/gr-comment/gr-comment_html.js
@@ -277,7 +277,10 @@
         ></gr-date-formatter>
       </span>
       <div class="show-hide">
-        <label class="show-hide">
+        <label
+          class="show-hide"
+          aria-label="[[_computeShowHideAriaLabel(collapsed)]]"
+        >
           <input
             type="checkbox"
             class="show-hide"