Review request (S): 7121373: Clean up CollectedHeap::is_in

[Stefan Karlsson]

http://cr.openjdk.java.net/~stefank/7121373/webrev.00

This change:
1) Fixes an assert in G1CollectedHeap::is_in when pointers below the 
heap was passed in as arguments.
2) Updates the comments to more correctly reflect how is_in is actually 
used/implemented.
3) Makes Space::is_in pure virtual, since the default implementation is 
never used.
4) Includes a unit test for CollectedHeap::is_in

StefanK