RFR: 8143251: HeapRetentionTest.java Test is failing on jdk9/dev

[Stefan Johansson]

Hi,

Please review this fix for:
https://bugs.openjdk.java.net/browse/JDK-8143251

Webrev:
http://cr.openjdk.java.net/~sjohanss/8143251/hotspot.00/

Summary:
After some refactoring and fixing in the G1 state machine it was 
possible to enter a state where we will ignore user requested concurrent 
GCs. This changes adds special treatment for the GC causes that are 
currently user requested to allow them to always trigger a new 
concurrent marking cycle.

Testing:
* JPRT
* RBT with the failing tests and additional GC tests.

Thanks,
Stefan