RFR: 8197852: Move G1DefaultPolicy into G1Policy

[Erik Helin]

Hi all,

this fairly small patch re-unites G1Policy and G1DefaultPolicy, there is 
no need for having both classes. With this patch we only have G1Policy 
(and of course G1CollectorPolicy as the little CollectorPolicy shim).

Issue:
https://bugs.openjdk.java.net/browse/JDK-8197852

Patch:
http://cr.openjdk.java.net/~ehelin/8197852/00/

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

Thanks,
Erik