Generate toCSSFooValue for CSSFunctionValue and CSSSVGDocumentValue

toCSSFooValue() will help to detect bad-casts as well as improve readibility.
To use DEFINE_CSS_VALUE_TYPE_CASTS, *CSS* is removed from isCSSSVGDocument()
as well as remove existing manual type cast functions.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/24954002

git-svn-id: svn://svn.chromium.org/blink/trunk@158419 bbb929c8-8fbe-4397-9dbb-9b2b20218538