Web Animations CSS: Fix crashing background-size

Test crashing due to merge fail between patches:
https://codereview.chromium.org/26259003/
https://codereview.chromium.org/26355006/

TBR=dstockwell

Review URL: https://codereview.chromium.org/27274003

git-svn-id: svn://svn.chromium.org/blink/trunk@159646 bbb929c8-8fbe-4397-9dbb-9b2b20218538