RFR 8065579: WB method to start G1 concurrent mark cycle should be introduced

[Leonid Mesnik]

Hi

Here is the fix which introduce WB method which stars concurrent mark 
cycle explicitly. It is needed to make CM cycle related tests more stable.

Changeset also includes test and very small fix of WB method 
g1InConcurrentMark .
Fix pass JPRT with hotspot testset.

JIRA: https://bugs.openjdk.java.net/browse/JDK-8065579
Webrev: http://cr.openjdk.java.net/~sjohanss/lmesnik/8065579/hotspot.00/
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