Makes windows repaint scrollbars when position changes

This is needed since arrows draw differently when thumb is at end
points.

BUG=358566
TEST=see bug
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/237943002

git-svn-id: svn://svn.chromium.org/blink/trunk@171683 bbb929c8-8fbe-4397-9dbb-9b2b20218538