Improve code generation for the HRandom instruction.

Improve code generation for the HRandom instruction.

Since the per-context random number generator is now
properly seeded upon context creation, we do not need
to check for lazy-initialization anymore, and so we
can implement the HRandom instruction w/o having to
call into the C function (which means we don't need
to MarkAsCall anymore).

TEST=cctest/test-random
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/23478031

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16684 ce2b1a6d-e550-0410-aec6-3dcde31c8c00