Do not arbitrarily snap scroll animation to 60Hz

Doing so artificially limits the frame rate and introduces
aliasing.

A complete solution would use a global animation timestamp
for the current frame, but that is left for a followup patch
to this one.

BUG=320955

Review URL: https://codereview.chromium.org/79093004

git-svn-id: svn://svn.chromium.org/blink/trunk@162563 bbb929c8-8fbe-4397-9dbb-9b2b20218538