Actually, GeckoView registers both desktop and mobile actor for `<select>`
element. It is unnecessary to use desktop's `<select>` actor.
But, if GeckoView doesn't register desktop's `<select>` actor,
`layout/forms/test/test_select_reframe.html` will be failure since GV doesn't
handle `mozhidedropdown` that is a event when the control is unbinded from
layout tree.
So we need to handle this for this situation that is one of dismissed cases.
Differential Revision: https://phabricator.services.mozilla.com/D149526
69e594563d