[MD-Settings-A11y] cr-expand-button should communicate that it expanded.

chrome/test/data/chromeos/oobe_webui_browsertest.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 GEN('#include "chrome/browser/ui/browser.h"');
 GEN('#include "chrome/browser/ui/browser_commands.h"');
 GEN('#include "chrome/browser/ui/exclusive_access/' +
     'fullscreen_controller_test.h"');
 
 /**
@@ skipping 59 matching lines @@
       '#supervised-user-creation-image-grid',
       'body > .decorated',
     ];
 
     // Enable when failure is resolved.
     // AX_ARIA_08: http://crbug.com/560924
     this.accessibilityAuditConfig.ignoreSelectors(
       'requiredOwnedAriaRoleMissing',
       requiredOwnedAriaRoleMissingSelectors);
 
+    // Polymer.PaperButtonBehavior.ariaActiveAttribute starts with aria- but is
+    // not an ARIA attribute. Ignore for paper-icon-button.
+    this.accessibilityAuditConfig.ignoreSelectors(
+      'badAriaAttribute',
+      'paper-icon-button[aria-active-attribute]');
+
     var badAriaAttributeSelectors = [
       '#forgotPasswordDlg',
       '#cancelConfirmDlg',
     ];
 
     // Enable when failure is resolved.
     // AX_ARIA_11: http://crbug.com/560935
     this.accessibilityAuditConfig.ignoreSelectors(
       'badAriaAttribute',
       badAriaAttributeSelectors);
@@ skipping 142 matching lines @@
       expectEquals(!!expectedClasses[className],
                    symbol.classList.contains(className));
     });
   });
 
   login.HIDDetectionScreen.contextChanged({
     'keyboard-state': 'connected'
   });
   expectTrue($('hid-keyboard-pincode').hidden);
 });