Move divider detection and creation onto TimelineUIUtils instance

BUG=361045
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/339493006

git-svn-id: svn://svn.chromium.org/blink/trunk@176493 bbb929c8-8fbe-4397-9dbb-9b2b20218538