Clean up dead code related to PopupContainer

The only type of PopupContainer created is PopupContainer::Select, so
there's no need for storing that type anywhere.

Also remove an always-false second argument from PopupContainer::popupOpened.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/159233003

git-svn-id: svn://svn.chromium.org/blink/trunk@166882 bbb929c8-8fbe-4397-9dbb-9b2b20218538