record object number in AgeTable for G1

[臧琳]

Hi all, 
        I recently worked with G1 and found that the AgeTable records the number of bytes for every age. But it is hard to get the object count for every age, I think it may be useful when I want to know how is the object distribution looks like for every age. so I made a patch to make AgeTable record the object count. 
        do you think it is a reasonable change?  
 

Thanks!
Lin