RFR(S/M): 7132678: G1: verify that the marking bitmaps have no marks for objects over TAMS

[John Cuthbertson]

Hi Everyone,

Can I have a couple of volunteers review the changes for this CR? The 
webrev can be found at: http://cr.openjdk.java.net/~johnc/7132678/webrev.0/

Most of the changes come from a patch that Tony gave me before he left 
and I had to tweak them slightly to remove a spurious failure. The 
changes verify that the heap regions don't have any marks between [TAMS, 
top) at strategic places: start and end of each GC, start and end of 
remark and cleanup, and when allocating a region. Tony deserves the bulk 
of the credit so, if possible and there are no objections, I intend to 
list him as author of the change and include myself as a reviewer.

Testing:
GC test suite with the both the new flags (separately and together) and 
a low IHOP value.
jprt with the new flags (+IgnoreUnrecognizedVMOptions so that product 
test runs did not fail).

Thanks,

JohnC