Generate toCSSFooValue() for CSSAspectRatioValue, CSSBorderImageSliceValue and CSSValueList

This cl cleanup static_cast<CSS*Value> type cast using DEFINE_CSS_VALUE_TYPE_CASTS.
toCSSFooValue() will help to detect bad-casts as well as improve readibility.

CSSAspectRatioValue, CSSBorderImageSliceValue are covered by this cl.

Additionally, CSSValueList uses DEFINE_CSS_VALUE_TYPE_CASTS instead of manual inline functions.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/23591068

git-svn-id: svn://svn.chromium.org/blink/trunk@158297 bbb929c8-8fbe-4397-9dbb-9b2b20218538