RFR(S) 8038404: Move object_iterate_mem from Space to CMS since it is only ever used by CMS

[Mikael Gerdin]

Hi,

I'm doing some cleanups in the "Space" classes to simplify the code for the G1 
class unloading work[1].
Here's a change to move CMS specific code to CompactibleFreeListSpace:

Bug: https://bugs.openjdk.java.net/browse/JDK-8038404
Webrev: http://cr.openjdk.java.net/~mgerdin/8038404/webrev.0

/Mikael

[1] http://openjdk.java.net/jeps/156