Revert "Web Animations CSS: Split AnimatableNumber into AnimatableDouble and AnimatableLength"

This reverts commit f01e9ff8366f3e6064b547761349339833282b27 -- broke build on all platforms.

TBR=alancutter,dstockwell,timloh@chromium.org,steveblock
BUG=257591

Review URL: https://codereview.chromium.org/25665004

git-svn-id: svn://svn.chromium.org/blink/trunk@158688 bbb929c8-8fbe-4397-9dbb-9b2b20218538