Web Animations: Add InterpolationEffect class.

This patch introduces the InterpolationEffect class, which represents a
collection of primitive animations along with local timing functions and
extents.

This class allows KeyframeEffectModels to be expressed in terms of
Interpolations. This code is not yet in use - see future patches.

BUG=346495

Review URL: https://codereview.chromium.org/183023017

git-svn-id: svn://svn.chromium.org/blink/trunk@168828 bbb929c8-8fbe-4397-9dbb-9b2b20218538