Fixed crash when interpolating -webkit-clip-path: none

The CSS value "none" was not handled correctly for animation of CSS
property -webkit-clip-path. Internally this value is stored as a null
ClipPathOperation pointer and led to a null deref.
This patch fixes the deref and correctly interprets null as CSSValueNone.

Review URL: https://codereview.chromium.org/68133004

git-svn-id: svn://svn.chromium.org/blink/trunk@161899 bbb929c8-8fbe-4397-9dbb-9b2b20218538