All places that use OS::Allocate either CHECK() that the result is non-NULL,
or use a reasonable fallback if they can't mmap memory.

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

Review URL: https://codereview.chromium.org/326323002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21810 ce2b1a6d-e550-0410-aec6-3dcde31c8c00