Web Animations CSS: Clamp interpolation of zoom at 0

Spec? https://developer.apple.com/library/safari/documentation/AppleApplications/Reference/SafariCSSRef/Articles/StandardCSSProperties.html

BUG=302104

Review URL: https://codereview.chromium.org/26601008

git-svn-id: svn://svn.chromium.org/blink/trunk@159550 bbb929c8-8fbe-4397-9dbb-9b2b20218538