Use toCSSFooValue() instead of using static_cast<CSSFooValue*> in CSSValue.cpp

Clean up static_cast<CSSFooValue*>.

BUG=309516

Review URL: https://codereview.chromium.org/205273003

git-svn-id: svn://svn.chromium.org/blink/trunk@169719 bbb929c8-8fbe-4397-9dbb-9b2b20218538