RFR: 8248133: SerialGC: Unify handling of all OopStorage instances in root processing

[Stefan Karlsson]

Hi all,

Please review this patch to nify handling of all OopStorage instances in 
root processing for the Serial GC.

https://cr.openjdk.java.net/~stefank/8248133/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8248133

This removes the explicit enumeration of "strong" OopStorages in the 
Serial GC. This is a step towards allowing the Runtime code to add new 
OopStorages without having to update all GCs.

Tested with tier1-tier3

Thanks,
StefanK