CSSFilterImageValue constructor should require both image and filter.

CSSFilterImageValue constructor should require both image and filter.
<https://webkit.org/b/125056>

Make the CSSFilterImageValue::create() helper take both the image and
filter CSSValues by PassRef since they should never be null.

Tweaked ComputedStyleExtractor::valueForFilter() to return a PassRef
for this to work.

Reviewed by Anders Carlsson.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@160541 268f45cc-cd09-0410-ab3c-d52691b4dbfc