Context menu should skip any menu construct whose submenu label is the empty string.

Specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#menu-item-generator
"3. Remove from output any menu construct whose submenu label is the empty string."

BUG=87553

Review URL: https://codereview.chromium.org/508853002

git-svn-id: svn://svn.chromium.org/blink/trunk@180936 bbb929c8-8fbe-4397-9dbb-9b2b20218538