RFR: 8199024: Remove unneccessary protected and virtual modifiers from G1CollectedHeap

[Erik Helin]

Hi all,

this tiny patch cleans up the visibility for members in G1CollectedHeap 
a bit. There is no need for protected members, I also removed two 
unnecessary virtual modifiers.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8199024

Patch:
http://cr.openjdk.java.net/~ehelin/8199024/00/

Testing:
- make run-test-tier1 on Linux, macOS, Windows (x86-64) and
                          Solaris (SPARC and x86-64)

Thanks,
Erik