WebUI - Error Console: Improve A11y by announcing expanded status.

Bug: 814117
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Iac4324eb5c8790edf9affec485f5a3a91c985908
Reviewed-on: https://chromium-review.googlesource.com/1068113
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560865}

chrome/browser/resources/md_extensions/error_page.html

@@ -191,9 +191,10 @@
               <div class="item-container">
                 <div class$="error-item
                     [[computeErrorClass_(item, selectedEntry_)]]">
-                  <div actionable class=" start" on-click="onErrorItemAction_"
+                  <div actionable class="start" on-click="onErrorItemAction_"
                       on-keydown="onErrorItemAction_" tabindex="0"
-                      role="button">
+                      role="button" aria-expanded$="[[isAriaExpanded_(
+                          index, selectedEntry_)]]">
                     <iron-icon icon$="[[computeErrorIcon_(item)]]"
                         title$="[[computeErrorTypeLabel_(item)]]">
                     </iron-icon>

chrome/browser/resources/md_extensions/error_page.js

@@ -254,11 +254,6 @@ cr.define('extensions', function() {
       return description;
     },
 
-    /** @private */
-    getExpandedClass_: function() {
-      return this.stackTraceExpanded_ ? 'expanded' : '';
-    },
-
     /**
      * @param {chrome.developerPrivate.StackFrame} frame
      * @return {string}
@@ -390,6 +385,16 @@ cr.define('extensions', function() {
       return index == this.selectedEntry_;
     },
 
+
+    /**
+     * @param {number} index
+     * @return {string} The aria-expanded value as a string.
+     * @private
+     */
+    isAriaExpanded_: function(index) {
+      return this.isOpened_(index).toString();
+    },
+
     /**
      * @param {!{type: string, code: string, model: !{index: number}}} e
      * @private