Fully repaint select popups when scrolled.

This used to be done by WebPopupMenuImpl::scroll but broke with
http://crrev.com/498593002.  Because the scrollbar and the area near it are
repainted, narrow popups scroll correctly, but wide popups (with very long
strings) do not.

Review URL: https://codereview.chromium.org/578453002

git-svn-id: svn://svn.chromium.org/blink/trunk@182087 bbb929c8-8fbe-4397-9dbb-9b2b20218538