Web Animations: Avoid a null dereference when compositor animation is discarded before start time is set

TBR=shanestephens
BUG=410429

Review URL: https://codereview.chromium.org/537073003

git-svn-id: svn://svn.chromium.org/blink/trunk@181354 bbb929c8-8fbe-4397-9dbb-9b2b20218538