Remove blank items from ExternalPopupMenu

This is a regression caused by https://codereview.chromium.org/415343003/
The list with "display: none" items should be trimmed, before being given
to the platform for display.
Modified test - https://codereview.chromium.org/475733002/

BUG=398051,403299
R=keishi@chromium.org

Review URL: https://codereview.chromium.org/475723002

git-svn-id: svn://svn.chromium.org/blink/trunk@180861 bbb929c8-8fbe-4397-9dbb-9b2b20218538