RFR (M): 8238855: Move G1ConcurrentMark flag sanity checks to g1Arguments

[Thomas Schatzl]

Hi all,

   can I have reviews for this change that moves (and deletes duplicate) 
flag checking from the G1ConcurrentMark class to the other G1 arguments 
processing?

Adds a test that checks whether the invariants before/after are still kept.

CR:
https://bugs.openjdk.java.net/browse/JDK-8238855
Webrev:
http://cr.openjdk.java.net/~tschatzl/8238855/webrev/
Testing:
hs-tier1-5 with new test

Thanks,
   Thomas