Allow proper highlighting on universal overflow scroll.

Universal accelerated overflow scroll doesn't paint
highlight gaps between scroll children because the
scrolling contents layer isn't always set up to
draw content.

BUG=381185,381187

Review URL: https://codereview.chromium.org/303253004

git-svn-id: svn://svn.chromium.org/blink/trunk@176243 bbb929c8-8fbe-4397-9dbb-9b2b20218538