Add the listbox earcon back to the popupbutton output rule.

TBR=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/1113843002

Cr-Commit-Position: refs/heads/master@{#327513}

chrome/browser/resources/chromeos/chromevox/cvox2/background/output.js

@@ -131,7 +131,8 @@ Output.ROLE_INFO_ = {
     msgId: 'aria_role_menu'
   },
   popUpButton: {
-    msgId: 'tag_button'
+    msgId: 'tag_button',
+    earcon: 'LISTBOX'
   },
   radioButton: {
     msgId: 'input_type_radio'