[8u60] Request for Approval: 8078414: Don't create impossible converters for ScriptObjectMirror

[Attila Szegedi]

Please approve.

Bug: https://bugs.openjdk.java.net/browse/JDK-8078414
jdk9 webrev: http://cr.openjdk.java.net/~attila/8078414/webrev.00
jdk9 review thread: http://mail.openjdk.java.net/pipermail/nashorn-dev/2015-May/004555.html

Changes apply cleanly to jdk8u-dev after path reshuffling from modular source code layout.

Thanks,
  Attila.