Web Animations: Add InterpolableValue hierarchy.

This patch introduces the InterpolableValue hierarchy, which stores
the interpolable portion of generic values.

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 header is not yet included anywhere. It will be used in future
patches.

BUG=346495

Review URL: https://codereview.chromium.org/177183007

git-svn-id: svn://svn.chromium.org/blink/trunk@168262 bbb929c8-8fbe-4397-9dbb-9b2b20218538