Define WebScrollOffsetAnimationCurve

This defines WebScrollOffsetAnimationCurve, needed for implementing
the CSSOM Smooth Scroll API.

The Chromium side of this is http://crrev.com/112933005.

BUG=243871

Review URL: https://codereview.chromium.org/365903004

git-svn-id: svn://svn.chromium.org/blink/trunk@177409 bbb929c8-8fbe-4397-9dbb-9b2b20218538