Web Animations CSS: Add support for building AnimatableLengths from calc lengths

This patch adds support for using calcs in animation keyframes under
the Web Animations implementation.

BUG=257591

Review URL: https://codereview.chromium.org/26971002

git-svn-id: svn://svn.chromium.org/blink/trunk@159517 bbb929c8-8fbe-4397-9dbb-9b2b20218538