Use DEFINE_CSS_VALUE_TYPE_CASTS instead of using manual type cast function for CSSShaderValue

This cl changes isCSSShaderValue() to isShaderValue() in order to use DEFINE_CSS_VALUE_TYPE_CASTS template.
Besides this cl checks if CSSValue is CSSShaderValue before doing a type cast so that it has same behavior with other
type cast function.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/24932003

git-svn-id: svn://svn.chromium.org/blink/trunk@158418 bbb929c8-8fbe-4397-9dbb-9b2b20218538