RFR: 8242448: Change HeapRegionManager::guarantee_contiguous_range to be assert_contiguous_range

[Stefan Johansson]

Hi,

Please review this follow-up to:
8241141: Restructure humongous object allocation in G1

Webreve: http://cr.openjdk.java.net/~sjohanss/8242448/00/
JBS: https://bugs.openjdk.java.net/browse/JDK-8242448

Summary:
During the review of 8241141 it was mentioned that the guarantee could 
be switched out to an assert, but the change was pushed without this 
change. Here is the switch.

Testing:
Sanity run in mach5.

Thanks,
Stefan