Web Animations: Avoid animating between null CSS image values

After a CSSImageSetValue is destructed the pointer to it from the
StyleFetchedImageSet is cleared. Calling
StyleFetchedImageSet::cssValue() after this point will return null.
Ideally we should reconstruct the original CSSImageSetvalue and
return that however there is not enough information in the
StyleFetchedImageSet to do this.

The code path to hit this condition is an obscure edge case
(see test) and the introduction of Oilpan should resolve the need to
nullify the CSSValue pointer in the first place.
With this in mind we should simply avoid crashing and introduce mildly
incorrect behaviour instead of adding complexity to the code while we
wait for Oilpan to complete.

BUG=363567

Review URL: https://codereview.chromium.org/255563002

git-svn-id: svn://svn.chromium.org/blink/trunk@172717 bbb929c8-8fbe-4397-9dbb-9b2b20218538