[MD-Settings A11y] Set alt text for expandable buttons.

ui/webui/resources/cr_elements/network/cr_network_select.html

Index: ui/webui/resources/cr_elements/network/cr_network_select.html
diff --git a/ui/webui/resources/cr_elements/network/cr_network_select.html b/ui/webui/resources/cr_elements/network/cr_network_select.html
index 4419cf16ccf5da6c5d28202bdbfc1defbdb53532..13bf5e77c14360b6ba1c2cc86cf8ab701c0249aa 100644
--- a/ui/webui/resources/cr_elements/network/cr_network_select.html
+++ b/ui/webui/resources/cr_elements/network/cr_network_select.html
@@ -19,6 +19,7 @@
             item="[[activeNetworkState]]">
         </cr-network-list-item>
         <cr-expand-button toggles hidden$="[[!expandable]]"
+            alt="Show available network connections"

[stevenjb, 2016/09/20 16:25:33]

We do have translations in this directory, see cr_network_list_item.js for an
example. There is a helper function in md_settings_localized_strings_provider.cc
(for now),  AddCrNetworkStrings(), that gets called by any WebUi using these
elements.

[hcarmona, 2016/09/22 14:57:12]

Fixed. I missed this label. I've looked through to make sure I didn't
miss i18n for any others.

             expanded="{{networkListOpened}}">
         </cr-expand-button>
       </div>