Web Animations: Fix setting of animatable value cache in StringKeyframe

Cache was not being set on the last frame, or when cloning.

Review URL: https://codereview.chromium.org/229393003

git-svn-id: svn://svn.chromium.org/blink/trunk@171092 bbb929c8-8fbe-4397-9dbb-9b2b20218538