Reparented overflow controls host layer must account for scrolling.

The math to position the host layer does not account for scrolling, and
this causes incorrect behavior with nested scrollers.

BUG=None

Review URL: https://codereview.chromium.org/667913003

git-svn-id: svn://svn.chromium.org/blink/trunk@185149 bbb929c8-8fbe-4397-9dbb-9b2b20218538