Web Animations CSS: Split AnimatableNumber into AnimatableDouble and AnimatableLength

AnimatableNumber currently does too many things. Splitting the class up
will allow us to subclass number functionality and make the overall
implementation cleaner.

BUG=257591

Review URL: https://codereview.chromium.org/25082007

git-svn-id: svn://svn.chromium.org/blink/trunk@158686 bbb929c8-8fbe-4397-9dbb-9b2b20218538