RFR (S): 8234000: Make HeapRegion::bottom/end/hrm_index const

[Thomas Schatzl]

Hi all,

   can I have reviews for this very small change that removes the 
possibility to change the bottom/end/hrm_index members in HeapRegion?

This is not required functionality, so I thought it would be good to 
clean up.

CR:
https://bugs.openjdk.java.net/browse/JDK-8234000
Webrev:
http://cr.openjdk.java.net/~tschatzl/8234000/webrev/
Testing:
local compilation, hs-tier1

Thanks,
   Thomas