RFR(M): 8236926: Concurrently uncommit memory in G1

[Liang Mao]

Hi Thomas/Stefan and other developers,

I have created the separate patch for 8236926. The concurrent work
has been moved to G1YoungRemSetSamplingThread according to previous
 comment. Specjvm2008 worked fine with the patch(specjbb2015 doesn't
 have the scenario of heap shrink). 
http://cr.openjdk.java.net/~luchsh/8236926.webrev/

Thanks,
Liang