As part of the review comments for 7092245, it was suggested that a more complete solution would be to redeclare HeapRegion::GrainBytes as a size_t. Currently it is declared as an int.
The size_t type is unsigned. So while it is unlikely we will need to be able to represent a heap region size > 2G in the next decade or so, changing the declaration will help type checking.
There are 2 other values associated with heap region size that should probably be changed from int to size_t too: GrainWords and CardsPerRegion.