32-bit Windows build fix

Unreviewed build fix.

* heap/SuperRegion.cpp:
* heap/SuperRegion.h: Use uint64_t instead of size_t.
(SuperRegion):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147329 268f45cc-cd09-0410-ab3c-d52691b4dbfc