DevTools: open context menu in the embedder even when no custom items are there.

BUG=424171
R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/670123002

git-svn-id: svn://svn.chromium.org/blink/trunk@184168 bbb929c8-8fbe-4397-9dbb-9b2b20218538