Add identifier for the localized text of a menulist

The text displayed should indicate the number of options selected.
e.g. "2 selected"

This CL is needed for other CLs to implement this change.

BUG=314704

Review URL: https://codereview.chromium.org/280483002

git-svn-id: svn://svn.chromium.org/blink/trunk@174267 bbb929c8-8fbe-4397-9dbb-9b2b20218538