Web Animations CSS: Check if animating element exists before calculating animation update

This was introduced in r161985 and causes LayoutTests to crash when Web Animations is enabled.

See https://codereview.chromium.org/71083002

R=timloh@chromium.org
BUG=232273, 314952, 319646

Review URL: https://codereview.chromium.org/73643003

git-svn-id: svn://svn.chromium.org/blink/trunk@162164 bbb929c8-8fbe-4397-9dbb-9b2b20218538