Use toCSSFooValue() generation macro for CSSInheritedValue, CSSVariableValue and CSSGridTemplateValue

This cl uses DEFINE_CSS_VALUE_TYPE_CASTS instead of using manual type cast functions.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/24906003

git-svn-id: svn://svn.chromium.org/blink/trunk@158422 bbb929c8-8fbe-4397-9dbb-9b2b20218538