Return 0 as maximal amount of physical memory if there is no limit.

That's what the comment says should happen, and what Heap's ctor
expects.

BUG=none
LOG=n
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/226113006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20583 ce2b1a6d-e550-0410-aec6-3dcde31c8c00