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.

This is a reland of http://crrev.com/25082007 after a change
conflict.

TBR=steveblock
NOTRY=TRUE
BUG=257591

Review URL: https://codereview.chromium.org/25678003

git-svn-id: svn://svn.chromium.org/blink/trunk@158768 bbb929c8-8fbe-4397-9dbb-9b2b20218538