Gardening: mark web-animations-api/timeline-time.html as flaky

TBR=ericwilligers@chromium.org
BUG=367903
NOTRY=true

Review URL: https://codereview.chromium.org/251133003

git-svn-id: svn://svn.chromium.org/blink/trunk@172791 bbb929c8-8fbe-4397-9dbb-9b2b20218538