Web Animations: Add InterpolableAnimatableValue class.

This patch introduces InterpolableAnimatableValues, which are
InterpolableValues that wrap AnimatableValues. In a future patch, these values
will be used to support the legacy AnimatableValue pipeline within the new
InterpolableValue pipeline, which means we won't need to convert all value
logic across at the same time.

BUG=346495

Review URL: https://codereview.chromium.org/192733002

git-svn-id: svn://svn.chromium.org/blink/trunk@168902 bbb929c8-8fbe-4397-9dbb-9b2b20218538