Do not use NSAnimation to drive scrollbar animations.

NSAnimation requires a connection to WindowServer. Since it's only really being
used to drive an animation timer, simply reimplement this with Blink primitives.

BUG=397642,306348,325938

Review URL: https://codereview.chromium.org/529103002

git-svn-id: svn://svn.chromium.org/blink/trunk@181388 bbb929c8-8fbe-4397-9dbb-9b2b20218538