[Extensions Toolbar Mac] Make context menus work for overflowed actions
On Mac, it doesn't look like nested menus (meaning a completely separate menu
running in an original menu, not to be mistaken with submenus) are supported.
This is very unfortunate. For overflowed toolbar actions, close the wrench menu,
then pop up the context menu over the wrench button (this is similar to what we
do for overflowed extension popups).
BUG=429810
Review URL: https://codereview.chromium.org/
821953004
Cr-Commit-Position: refs/heads/master@{#312013}