Review request for JDK-8074661

[Attila Szegedi]

Please review JDK-8074661 at 
<http://cr.openjdk.java.net/~attila/8074661/webrev.00> for 
<https://bugs.openjdk.java.net/browse/JDK-8074661>

This change adds the AbstractJSObject.getDefaultValue() method to jdk9 codebase 
so that it's compatible with the jdk8u backport of JDK-8072426 (which had to 
introduce it).

Thanks,
  Attila.