Review request for JDK-8146274: Thread spinning on WeakHashMap.getEntry() with concurrent use of nashorn

[Hannes Wallnoefer]

Please review:

Webrev: http://cr.openjdk.java.net/~hannesw/8146274/
Bug: https://bugs.openjdk.java.net/browse/JDK-8146274
I was not able to reproduce the issue although I tried my best to follow 
the instructions. However, it is pretty clear that the issue is that the 
cause for the concurrent modification exception is the sharing of  
WeakPropertyMapSets between PropertyMapListeners.

Thanks,
Hannes