Start service scroll animation on non-frame scrollable areas

PageAnimator used to only service scroll animation on FrameViews. This
patch added support to service scroll animation on non frame
scrollable areas as well.

This was part of CL 214953004, which got reverted in 335103004 when it
caused crbug.com/383946. I am adding back the reverted CL in parts. The
bug was caused by enable smooth scrolling on non frame scrollable
areas, thus though this CL added support, smooth scrolling on non
frame scrollable areas are not turned on in this patch.

BUG=299059

Review URL: https://codereview.chromium.org/483773003

git-svn-id: svn://svn.chromium.org/blink/trunk@180734 bbb929c8-8fbe-4397-9dbb-9b2b20218538