RFR [1/2] 8065358: JDK-8065218 Move CMS-specific fields from Space to CompactibleFreeListSpace

[Mikael Gerdin]

Hi all,

This is the first patch of two to fix JDK-8065358.

In this change I suggest that we move a field used only by CMS to 
CompactibleFreeListSpace and introduce a virtual getter in the Space 
base class.

I estimate that the performance impact of the virtual call will be small 
since the getter path already does several virtual calls.

Moving the field will allow me to add another field to G1's 
G1OffsetTableContigSpace in patch [2/2] without introducing a footprint 
regression.

Webrev: http://cr.openjdk.java.net/~mgerdin/8065218/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8065218
Parent bug: https://bugs.openjdk.java.net/browse/JDK-8065358
Testing: JPRT

Thanks
/Mikael