Fix crash when interpolating auto shape values

The CSS value "auto" was not handled correctly for animation of CSS
properties shape-inside or shape-outside. Internally this value is stored
as a null ShapeValue pointer and led to a null deref.
This patch fixes the deref and correctly interprets null as CSSValueAuto.

Review URL: https://codereview.chromium.org/68563002

git-svn-id: svn://svn.chromium.org/blink/trunk@161788 bbb929c8-8fbe-4397-9dbb-9b2b20218538