Merge

6820066: Check that -XX:ParGCArrayScanChunk has a value larger than zero. Summary: Check that -XX:ParGCArrayScanChunk has a value larger than zero. Reviewed-by: johnc, jmasa, ysr

6987703: iCMS: Intermittent hang with gc/gctests/CallGC/CallGC01 and +ExplicitGCInvokesConcurrent Summary: Count enable_icms() and disable_icms() events so as to prevent inteference between concurrent calls, which can cause the iCMS thread to be left stranded in icms_wait() with an unserviced request and no young allocations to unwedge it. Reviewed-by: jcoomes, poonam