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

ui/webui/resources/cr_elements/cr_expand_button/cr_expand_button.html

Index: ui/webui/resources/cr_elements/cr_expand_button/cr_expand_button.html
diff --git a/ui/webui/resources/cr_elements/cr_expand_button/cr_expand_button.html b/ui/webui/resources/cr_elements/cr_expand_button/cr_expand_button.html
index 4e5047b0c6f74227ae7b724bdbcf0df641817cd2..a3f9fd8447eb831030998025843c6fa0908a19d6 100644
--- a/ui/webui/resources/cr_elements/cr_expand_button/cr_expand_button.html
+++ b/ui/webui/resources/cr_elements/cr_expand_button/cr_expand_button.html
@@ -8,7 +8,7 @@
   <template>
     <content></content>
     <paper-icon-button toggles active="{{expanded}}" disabled="[[disabled]]"
-        icon="[[iconName_(expanded)]]">
+        icon="[[iconName_(expanded)]]" aria-active-attribute="aria-expanded">

[michaelpg, 2016/09/07 22:38:42]

shouldn't this reference *what* it expands?
https://www.w3.org/TR/wai-aria/states_and_properties#aria-expanded

How do we refer to an element that isn't in our shadow root? :-\

[hcarmona, 2016/09/26 18:22:51]

Only one screen reader supports this property and not all aria
properties work well with shadow dom. (this is one of the ones) that
doesn't work well. :-/

I've filed http://crbug.com/650334 to track adding the aria-controls
attribute after speaking with dmazzoni@ about this.

     </paper-icon-button>
   </template>
   <script src="cr_expand_button.js"></script>