RFR: 8199850: Move parsing of VerifyGCType to G1

[Per Liden]

When JDK-8191821 introduced the VerifyGCType option for G1 I didn't 
quite see the need for this to affect the GCArguments class. I'm 
proposing we move this logic into G1HeapVerifier, which I believe is a 
better place for this.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199850
Webrev: http://cr.openjdk.java.net/~pliden/8199850/webrev.0

Testing: Used gc/g1/TestVerifyGCType.java to verify that nothing broke

/Per