Add test_runner support for new accessibility event

Depends on: https://codereview.chromium.org/1020473004

BUG=323462

Review URL: https://codereview.chromium.org/1015403002

Cr-Commit-Position: refs/heads/master@{#322583}

content/shell/renderer/test_runner/web_test_proxy.cc

@@ -783,6 +783,9 @@ void WebTestProxyBase::PostAccessibilityEvent(const blink::WebAXObject& obj,
     case blink::WebAXEventMenuListItemSelected:
       event_name = "MenuListItemSelected";
       break;
+    case blink::WebAXEventMenuListItemUnselected:
+      event_name = "MenuListItemUnselected";
+      break;
     case blink::WebAXEventMenuListValueChanged:
       event_name = "MenuListValueChanged";
       break;