Introduce CSS_VALUE_TYPE_CASTS macro to cast CSSValue

As SVG and HTML have type cast function, CSSValue also can have it. This cl introduces
CSS_VALUE_TYPE_CASTS macro to make type cast function easily. This type cast macros
will help to detect bad-cast bugs as well as improve a code readibility.

This patch adds the following methods,

 - CSSFooValue* toCSSFooValue(CSSValue*)
 - const CSSFooValue* toCSSFooValue(const CSSValue*)

This cl applies the macro to CSSReflectValue and CSSImageSetValue first.

Besides this cl adds these functions to detect unnecessary type casting.
  - void toFooValue(const CSS*Value*)
  - void toFooValue(const CSS*Value&)

This will let us know where does unnecessary type casting happen in compilation time.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/24020008

git-svn-id: svn://svn.chromium.org/blink/trunk@158254 bbb929c8-8fbe-4397-9dbb-9b2b20218538