RFR (S): 8243974: Move G1CollectedHeap::millis_since_last_gc support from G1Policy

[Thomas Schatzl]

Hi all,

   please review the following change that moves 
G1CollectedHeap::millis_since_last_gc support away from G1Policy to 
G1CollectedHeap. That functionality has not much to do with policy 
decisions at all.

Fwiw, found the issue 8248221 when fixing this. Will send out that one soon.

Based on 8165501.

CR:
https://bugs.openjdk.java.net/browse/JDK-8243974
Webrev:
http://cr.openjdk.java.net/~tschatzl/8243974/webrev/
Testing:
tier1-5 with 8165501, 8248221; see also 8248221 for manual testing info

Thanks,
   Thomas