Web Animations: Add Interpolation class.

This patch introduces the Interpolation class, which represents
a primitive animation between two values (expressed as InterpolableValues).

This is part of an effort to refactor the animations pipeline in order
to make it generic on animation value type (see linked master bug).

This code is not yet used anywhere. It will be used in future patches.

BUG=346495

Review URL: https://codereview.chromium.org/182383011

git-svn-id: svn://svn.chromium.org/blink/trunk@168629 bbb929c8-8fbe-4397-9dbb-9b2b20218538