Allow the embedder to pass the virtual memory limit to v8

The getrlimit() call might be sandboxed, so it's not safe to use it.

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

Review URL: https://codereview.chromium.org/228923002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20615 ce2b1a6d-e550-0410-aec6-3dcde31c8c00