[EFL][WK2] Fix build break after r140891.

Unreviewed build fix. SerializedScriptValue needs to use PassRefPtr in WK2 as well.

* Shared/WebSerializedScriptValue.h:
(WebKit::WebSerializedScriptValue::create):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140902 268f45cc-cd09-0410-ab3c-d52691b4dbfc