Clamp scroll offset to min/max in PopupListBox::updateScrollbars.

This was previously done in ScrollView::setScrollOffsetFromUpdateScrollbars but
got lost with http://crrev.com/598093008.

BUG=424042

Review URL: https://codereview.chromium.org/662733002

git-svn-id: svn://svn.chromium.org/blink/trunk@183870 bbb929c8-8fbe-4397-9dbb-9b2b20218538