ASSERT: Bad cast from CSSInitialValue to CSSValueList

The issue was that we would cast to CSSValueList without checking
the type of the CSSValue. After this change, we use the ASSERT'ing
cast and explicitly check the type of the CSSValue before the cast.

BUG=279286

Review URL: https://chromiumcodereview.appspot.com/23604036

git-svn-id: svn://svn.chromium.org/blink/trunk@157255 bbb929c8-8fbe-4397-9dbb-9b2b20218538