Check that the local frame is not null before instantiating AnimationWorklet.

This patch fixes the crash in blink::LocalFrame::document.
See https://cluster-fuzz.appspot.com/testcase?key=5879802211074048 for more details.

This is simlar to what we already do in WindowPaintWorklet::paintWorklet.

BUG=567358,634253

Review-Url: https://codereview.chromium.org/2212003002
Cr-Commit-Position: refs/heads/master@{#409973}
1 file changed