RFR(s): 8068344: G1GC concurrent marking time report is not accurate

[sangheon]

Hi all,

Please review this change to enhance G1 concurrent mark time report.

This patch includes 2 changes.
1. Include cleanup time at virtual time of ConcurrentMarkThread class: 
Looking at the comment, cleanup was expected to be small enough to be 
ignored but actually it isn't. And this CR is requesting to include it.

2. Remove '_init_mark' which is not used.

This CR also mentioned about 2 more items but #3 is resulted from 
misunderstanding of the log and #4 is already fixed.

CR: https://bugs.openjdk.java.net/browse/JDK-8068344
Webrev: http://cr.openjdk.java.net/~sangheki/8068344/webrev.00
Testing: JPRT, local test to measure the time report

Thanks,
Sangheon