RFR(S): 8194736: Refactor weak oops in ProtectionDomain table to use the Access API

[Erik Österlund]

Hi,

Like other tables containing weak oop references, the ProtectionDomain 
table should use the Access API.
This is a patch that does that, in a fashion very similar to what has 
been done to other weak tables recently.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8194736

Webrev:
http://cr.openjdk.java.net/~eosterlund/8194736/webrev.00/

Thanks,
/Erik