Web Animations CSS: Fix crashing -webkit-mask-box-image test

Special cased animated shorthand properties to depend on the Web Animations
runtime flag.
virtual/web-animations-css/animations/cross-fade-webkit-mask-box-image.html
now has image failure instead of crashing.
Image failure due to: crbug.com/309962

BUG=257591

Review URL: https://codereview.chromium.org/32193003

git-svn-id: svn://svn.chromium.org/blink/trunk@160180 bbb929c8-8fbe-4397-9dbb-9b2b20218538