Web Animations CSS: Check if animating element exists before calculating animation update
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@162164 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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