Styling option should not override option:checked UA style

This adds !important to option:checked styles, so that styling option won't override the selected appearance.
This adds -internal-listbox which applies to listbox mode select elements.
We need it because we can't remove the !important styles for select[size=0], select[size=1]

BUG=398417

Review URL: https://codereview.chromium.org/450623002

git-svn-id: svn://svn.chromium.org/blink/trunk@179782 bbb929c8-8fbe-4397-9dbb-9b2b20218538