Remove dead plumbing for ScrollAnimatorMac.

Both of these arguments were dead code.  Unclear if
this whole infrastructure is.  ScrollAnimatorMac
appears to be listening for overlay vs. not style
changes.

I ran into this while looking into fixing crbug.com/365509

BUG=365509

Review URL: https://codereview.chromium.org/247863002

git-svn-id: svn://svn.chromium.org/blink/trunk@172306 bbb929c8-8fbe-4397-9dbb-9b2b20218538