WebUI: Add toolbar role and aria-label to multi-select toolbar

Bug: 1142076
Change-Id: I81cee8a975a7d289fd4272475f9010aa87c909c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2559289Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831131}

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_selection_overlay.js

@@ -42,6 +42,10 @@ Polymer({
     'updateSelectionLabel_(show, selectionLabel)',
   ],
 
+  hostAttributes: {
+    'role': 'toolbar',
+  },
+
   /** @return {HTMLElement} */
   get deleteButton() {
     return /** @type {HTMLElement} */ (this.$$('#delete'));
@@ -64,6 +68,7 @@ Polymer({
     this.debounce('updateSelectionLabel_', () => {
       this.selectionLabel_ =
           this.show ? this.selectionLabel : this.selectionLabel_;
+      this.setAttribute('aria-label', this.selectionLabel_);
     });
   },