Introduce ScriptValue::createUndefined

Add ScriptValue::createUndefined static function which returns
a ScriptValue representing an undefined value (not an empty value).

BUG=320984
R=haraken

Review URL: https://codereview.chromium.org/76083002

git-svn-id: svn://svn.chromium.org/blink/trunk@162292 bbb929c8-8fbe-4397-9dbb-9b2b20218538