RFR: 8142495: Move G1ParEvacuateFollowersClosure to header file

[Erik Helin]

Hi all,

this small patch just moves the class G1ParEvacuateFollowersClosure from
g1CollectedHeap.cpp to g1CollectedHeap.hpp so that it can be instatiated
from other files.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8142495

Webrev:
http://cr.openjdk.java.net/~ehelin/8142495/webrev.00/

Testing:
JPRT

Thanks,
Erik