Use toCSSFooValue() instead of using static_cast<const CSSFooValue*>

Cleanup static_cast<const CSSFooValue*> in order to detect bad type casts.

BUG=N/A

Review URL: https://codereview.chromium.org/26623002

git-svn-id: svn://svn.chromium.org/blink/trunk@159212 bbb929c8-8fbe-4397-9dbb-9b2b20218538