Remove CSSValueListInspector

The CSSValueListInspector class doesn't add anything to the CSSValueList
interface. The behaviour of interpreting a non-CSSValueList as a list of
zero length isn't in general useful. This was only being used in one
location (the StyleBuilder for text-decoration-line), which I've made
explicitly check the type.

Review URL: https://codereview.chromium.org/613713003

git-svn-id: svn://svn.chromium.org/blink/trunk@183022 bbb929c8-8fbe-4397-9dbb-9b2b20218538