RFR (S): 8165501: Serial, Parallel and G1 should track the time since last gc for millis_since_last_gc() with full precision

[Thomas Schatzl]

Hi all,

   can I get reviews for this small change that unifies code a bit 
between Serial, Parallel and G1 with regards to tracking the time stamp 
for CollectedHeap::millis_since_last_gc?

In particular, it names the members uniformly, and removes an 
unnecessary division by internally storing raw os::JavaTimeNanos instead 
of some intermediate result.

Other collectors are not affected because they calculate 
millis_since_last_gc from an internal clock source.

There is no functionality change

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

Thanks,
   Thomas