Web Animations CSS: Support animation of zoom

Preserves the existing behavior where animation of zoom can produce values
outside the valid range.

BUG=257591, 302104

Review URL: https://codereview.chromium.org/25197002

git-svn-id: svn://svn.chromium.org/blink/trunk@158797 bbb929c8-8fbe-4397-9dbb-9b2b20218538