[Extensions a11y] Fix screen reader focusing on invisible toast text

aria-hidden=true is set on the toast when it's hidden, and removed right
before the toast is shown.

Tested manually for chrome://extensions on Mac using VoiceOver

Bug: 1084193
Change-Id: I5f11f5beec4facb52a6ab41fc34bc8b2a9d33651
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2559733Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832127}

ui/webui/resources/cr_elements/cr_toast/cr_toast.js

@@ -38,7 +38,7 @@ Polymer({
 
     if (this.open && this.duration !== 0) {
       this.hideTimeoutId_ = window.setTimeout(() => {
-        this._setOpen(false);
+        this.hide();
       }, this.duration);
     }
   },
@@ -59,6 +59,10 @@ Polymer({
     // the same as a previous toast.
     this.removeAttribute('role');
 
+    // Reset the aria-hidden attribute as screen readers need to access the
+    // contents of an opened toast.
+    this.removeAttribute('aria-hidden');
+
     this._setOpen(true);
     this.setAttribute('role', 'alert');
 
@@ -67,8 +71,12 @@ Polymer({
     }
   },
 
-  /** Hides the toast. */
+  /**
+   * Hides the toast and ensures that screen readers cannot its contents while
+   * hidden.
+   */
   hide() {
+    this.setAttribute('aria-hidden', 'true');
     this._setOpen(false);
   },
 });